History of Vaccination: Eradication of Smallpox and Polio
This is a three-part series on vaccination. In the first part, we will describe the mechanism of vaccination. We will review the history of vaccination and its predecessor, inoculation. We will discuss at length the development of vaccination techniques, and world-wide campaigns to eradicate various contagious diseases. We will review the history of two diseases where vaccination has been most effective: smallpox and poliomyelitis. Smallpox has been completely eradicated, and at present polio appears to be in an “end-stage” where complete eradication could be achieved in the coming decade.
We also introduce the concept of “herd immunity.” We discuss this concept and show that for a communicable disease with an effective vaccine, a critical number is the “herd immunity threshold” or HIT. When the percentage of the population with immunity to the disease exceeds the HIT, then that disease will not be able to produce an epidemic in the population, and the disease will die out.
At the end of part I, we will briefly review other contagious diseases such as measles and mumps that are currently subject to extensive campaigns of vaccination. The success of these vaccination campaigns means that diseases such as measles and rubella have become much less common. In fact, there are no longer endemic cases of measles in the U.S. – the most recent outbreaks of measles have resulted from people who were infected abroad, and then returned to communities where the vaccination rate was lower than normal.
Vaccination is an exceptionally successful process that provides protection against a number of infectious diseases. As we will show, vaccination campaigns have shielded billions of people around the globe from the effects of some extremely harmful diseases. For example, the entire population of Earth is now safe from the deadly and disfiguring disease of smallpox. Vaccination is arguably the most effective public health medical procedure ever devised.
Currently there exist vaccines for about 25 infectious diseases. At the moment, a major problem in the developed world is that many people are choosing not to vaccinate their children, to wait until children are older before vaccinating, or to “space” vaccinations at wider intervals. Calandrillo (U. Mich. J.L. Reform 353, 419 (2004)) has estimated that “vaccine-preventable diseases impose $10 billion worth of healthcare costs and over 30,000 otherwise avoidable deaths in America each year.”
Ever since its inception, the process of vaccination has been a controversial topic. In Part II, we review the history of controversy over vaccination. We assess some of the original arguments against vaccination, or against efforts to make vaccination compulsory throughout a population. In recent years an increasingly large number of people in the U.S. have chosen not to vaccinate their children. Paradoxically, the tremendous success of vaccination campaigns may have contributed to this. The current generation of parents are sufficiently young that they have no personal familiarity with people who suffered or died from smallpox or polio, or even more common contagious diseases such as measles or mumps.
In Part II of this series we will analyze survey data and arguments for or against vaccination. We will present our conclusions about this issue, with a particular focus on young parents who may be deciding whether or not to have their children vaccinated.
Much of the current controversy over vaccination is centered around groups of people who oppose vaccination. More recently there are groups that claim that vaccination is a cause of other conditions such as autism. In Part III, we present capsule biographies of leading “anti-vaxers.” We discuss their positions and assess the validity of their arguments.
A. The History of Vaccination and Application to Smallpox
Vaccination refers to the process by which a vaccine, or an antigenic material, is administered to an individual. Vaccination stimulates the recipient to develop immunity to a particular disease.
The biomolecular science by which the vaccination builds up immunity is well understood as the result of research on the generation of antibody genes that led to and followed Susumu Tonegawa’s 1987 Nobel Prize in Physiology and Medicine. Antibodies are specific proteins attached to the surface of particular white blood cells known as B lymphocytes, and which are capable of binding to the antigen molecules of a particular invading virus or bacterial infection. The body can produce an enormous variety of antibodies by random selection of gene segments within each lymphocyte cell. When antigen molecules bind to B lymphocyte cells, the antigens themselves become tagged for later recognition and destruction by other white blood cells, but they trigger rapid multiplication of the lymphocyte cells into daughter clones, each of which expresses the same antibody, except for random mutations.
Antibodies against a specific antigen are not found in significant amounts in blood samples prior to vaccination with a weakened form of the antigen. But in response to the vaccination, pre-existing antibodies that may have weak affinity to bind to the invading antigen multiply and mutate during the rapid reproduction of the lymphocyte cells. These mutations are naturally selected by the ability of their generated antibodies to bind to the injected antigens; mutations that weaken this ability die off. This process of mutation and natural selection leads to rapid growth in the number and binding affinity, hence, in the efficacy of the antibodies. Both types of improvement often occur even more rapidly after a subsequent booster vaccination with the same weakened antigen. Some fraction of the lymphocyte cells with the advanced antibodies that have successfully cleared antigens from the body will remain in the body to launch a far more efficient battle against subsequent invasions by the same antigen. Those remaining antibodies fuel immunity to the disease.
At present vaccines exist in several different forms. As described by Aaron Rothstein, “some vaccines contain a killed version of a germ. Chemicals like formaldehyde inactivate the viruses or bacteria, rendering them unable to replicate but leaving their surface proteins intact so that the immune system can recognize them. (Jonas Salk’s famous polio vaccine is an example of this kind of “inactivated” vaccine.) Another type of vaccine contains a weakened but active form of the pathogen. (Examples of such “attenuated” or “live” vaccines include the measles, mumps, and rubella vaccines.) A third kind of vaccine helps the immune system respond not to disease-causing pathogens but to the toxins they produce. (The vaccine for tetanus is an example of this ‘toxoid’ type; it teaches the body how to destroy not the tetanus bacteria but the toxin produced by the bacteria.) Other types expose the body to part or parts of a harmful pathogen (such as the injected vaccines for the flu and for hepatitis B).”
Long before the biomolecular basis of immunity was understood, the first widespread use of inoculation and vaccination techniques was against the contagious disease smallpox. The terrible effects of smallpox, and the crusade to prevent and eventually eradicate the disease, plays such a central role in the history of vaccination that we will first summarize the disease and its effects. Next, we will review the efforts to prevent the disease by conferring immunity on the population. Here, we will highlight the efforts of the 18th-century British physician Edward Jenner to develop a safe and effective vaccine against smallpox. Finally, we will review the efforts in the 20th century that have led to the eradication of smallpox.
Before the advent of vaccination, smallpox was an exceptionally serious disease. It was fatal to between 20% and 60% of adults who suffered from it; and it was fatal to nearly 80% of children. Smallpox is caused by one of two variants of the Variola virus, Variola major and Variola minor. The initial symptoms of the disease – muscle pain, headache and general weakness – are similar to other viral diseases such as influenza and the common cold.
Variola is “a large brick-shaped virus measuring approximately 302 to 350 nanometers by 244 to 270 nm, with a single linear double stranded DNA genome 186 kilobase pairs (kbp) in size and containing a hairpin loop at each end.” Figure 1.1 shows a transmission electron micrograph photo of smallpox virions.
Variola is one of four orthopoxviruses that can cause infection in humans; the other three are vaccinia, cowpox and monkeypox. Poxviruses have the unique property that they reproduce in the cytoplasm of the cell, rather than in the nucleus.
Once a person has become infected, after 12-15 days the first visible lesions appear on the mucous membranes in the region of the mouth and throat. The lesions rapidly enlarge and then rupture, which releases large amounts of virus into the patient’s saliva.
The virus acts predominantly on the skin cells, and the patient’s body becomes covered with fluid-filled bumps. The bumps produce scabs that eventually fall off, leaving permanent scars. A number of people who contract smallpox become blind. Figure 1.2 below shows a child who had been infected with smallpox before its world-wide eradication in 1980.
Smallpox is generally transmitted from person to person by airborne droplets from an infected individual. It can also be transmitted through bodily fluids or through infected objects such as clothing or bedding. Although smallpox is highly contagious, it does not spread as rapidly as some other contagious diseases, because transmission of the disease generally requires close contact with an infected individual.
There are four variants of smallpox. “Ordinary” smallpox constitutes roughly 90% of smallpox in unvaccinated persons. A version of ordinary smallpox is called confluent smallpox. In these cases, the smallpox blisters merge into sheets; the resulting “confluent” rash begins to detach the outer layers of skin. Confluent smallpox is frequently fatal.
A second variant is called “modified” smallpox. This is generally found in people who have been previously vaccinated against the disease. Patients with the modified variant have fewer lesions and they tend to be more superficial. There have been no known fatalities from modified smallpox.
A third variant is called “malignant” smallpox. This variant occurs in 5-10% of smallpox victims, and roughly 72% of those suffering from malignant smallpox are children. In the malignant variety, the vesicles “contained little fluid, were soft and velvety to the touch, and may have contained hemorrhages.” It is not known why some patients suffered from the malignant variety of smallpox, but it was nearly always fatal.
The final variant was called “hemorrhagic” smallpox. In the hemorrhagic variant, before the smallpox rash appears on the skin, the patient suffers extensive bleeding into the skin and gastrointestinal tract. The patient’s skin thus appears black or charred, and this has led to the name “black pox” for this variant. It occurs primarily in adults. Figure 1.3 shows a photo of an individual suffering from hemorrhagic smallpox. This variant was generally fatal.
Since both smallpox and chickenpox produce rashes on the body, it was possible to mistake the two diseases. However, the rash produced by chickenpox is predominantly confined to the torso, while the rash from smallpox occurred much more frequently on the patient’s limbs. Figure 1.4 below shows the distinct difference in distribution of the rash between smallpox and chickenpox.
The history of smallpox is somewhat difficult to determine because of our inability to definitively distinguish it from similar diseases. The Variola virus is believed to have evolved from an earlier virus, one that affected African rodents. Evidence of smallpox has been found in Egyptian mummies from 3 BC, which suggests that it appeared as a human disease between 3,000-4,000 years ago.
We know that smallpox-like diseases have existed for a few millennia in Egypt. One theory is that smallpox was introduced into India from Egypt in the first millennium BCE, and from there it made its way into China in the first century AD, and subsequently from China to Japan in the 6th century. In 8th-century Japan, an epidemic reputed to be smallpox killed as much as one-third of the population over a two-year period.
In ancient societies, several deities were believed to cause smallpox, and were worshipped in the hope that they would confer immunity against the disease. This included the Yoruba god Sopona and the Hindu goddess of smallpox, Sitala Mata.
In Europe and south-west Asia, smallpox seems to have appeared at a significantly later date. There is no clear-cut description of smallpox in the Bible, and no description of this disease in the collected works of Hippocrates (460-370 BCE). Epidemics that might have been caused by smallpox occurred in 165-180 AD in the Roman Empire, and in the 5th and 6th centuries in Europe.
The Persian physician Rhazes provided a definitive description of smallpox in his 9th-century treatise The Book of Smallpox and Measles. In that book he also described how to differentiate chickenpox from smallpox. By the time of the European voyages of exploration and colonization, smallpox was well established in western Europe. Until the end of the 15th century, we have no record of smallpox existing in the New World.
The early European voyagers to North and South America brought several new infectious diseases with them, most importantly smallpox. That disease was introduced to the Caribbean island of Hispaniola in 1509 and to the mainland of Mexico in 1520. Smallpox took a terrible toll on the Amerindian population. Apparently that disease played an important role in the defeat of the Incas and the Aztecs by the Spaniards.
Native American tribes in North America also were devastated by smallpox. It has been reported that in the early American colonies, the fatality rates for Native Americans from smallpox epidemics were as high as 80-90%.
A number of famous individuals either contracted smallpox or died from that disease. It is believed that Ramses V of Egypt perished from the disease, as did China’s Shunzhi Emperor and Tongzhi Emperor. The ruler of the Aztec city of Tenochtitlan, Cuitlahuac, died from smallpox shortly after its introduction to the Americas, as did the Incan emperor Huayna Capac.
The English king Henry VIII’s fourth wife, Anne of Cleves, contracted smallpox and was scarred but survived, as did two of his daughters, Mary I of England and Elizabeth I of England; and Henry’s only son Edward VI died from complications shortly after it appeared he had recovered from smallpox. Three American presidents contracted smallpox and survived – George Washington, Andrew Jackson and Abraham Lincoln. The Soviet leader Joseph Stalin contracted smallpox as a child, which left his face badly scarred.
In the next sections we will review the history of inoculation and vaccination techniques to combat smallpox. By the end of the 19th century, widespread vaccination in the United States and Europe had significantly decreased the occurrence of this disease. However, throughout the rest of the world, smallpox persisted well into the 20th century. After the final eradication of smallpox was announced by the World Health Organization in 1980, it was estimated that in the 20th century alone, 300-500 million people had died from the disease.
2) The Treatment of Smallpox
The earliest effective method against smallpox was called inoculation or variolation. That procedure involved inserting matter from one individual under the skin of a second individual. One method was to take powdered smallpox scabs or fluid from pustules from someone suffering from the disease. This method transmitted live smallpox pathogens into the patient. A second method was to take materials from a person who had recently been variolated, and insert that material into the patient.
After inoculation, the patient would develop a (generally) mild case of smallpox. Following the course of this hopefully mild disease, the patient would have developed immunity to smallpox. Although inoculation had the possibility of conferring immunity against the disease, in some cases the inoculated person could contract smallpox, and occasionally the recipient would suffer a severe or even fatal case of this disease. Also, it was possible for the inoculated individual to pass the disease on to others.
Apparently inoculation procedures have been used for centuries. For example, the Chinese took scabs from smallpox patients who experienced a mild case of the disease. They ground those scabs up to produce a powder; that powder was then blown into the nostrils of children. Such a procedure was called insufflation, and had the same effect as inoculation. There are claims that inoculation had been used even earlier in India, but we know of no firm evidence that this was the case.
By the 15th century, the Chinese had developed a rather elaborate ritual for inoculation. The blowpipe was made of silver, and material was blown into the right nostril of boys and the left nostril of girls. After insufflation, the children were subsequently quarantined from the general population until their rash had cleared.
British physician Edward Jenner is acknowledged as the father of modern vaccination, and we will review Jenner’s contribution in the next section. However, at the time of his breakthrough Jenner was building on knowledge provided to him by several earlier Europeans. In 1700 Dr. Mark Lister provided a report to the British Royal Society on the Chinese practice of insufflation. Then in 1717 Lady Mary Wortley Montagu, the wife of the British Ambassador to the Ottoman Empire, wrote a letter to a friend in which she praised the technique of variolation that was widely used in Constantinople.
Lady Mary’s brother had died from smallpox, and she herself had contracted the disease, which nearly killed her and left her with severe facial scarring. In 1717, she had her son variolated in Constantinople under the supervision of the Embassy doctor. After returning to England, she had her daughter variolated in the presence of physicians of the Royal Court.
The process of variolation became widely prevalent in England during the 18th century. It was initially highly controversial, in part because the early physicians who carried it out accompanied it by severe blood-letting, which was justified on the basis that it was necessary to ‘purify’ the blood. Also, there were a few highly-publicized failures of this technique; in particular, one of the children of King George III of Britain died following variolation in 1783.
However, in the mid-18th century Dr. Robert Sutton developed a highly reliable protocol for variolation. Sutton kept his method secret and passed it down only to his three sons. Sutton’s techniques were so effective that the family developed a franchise of enormously successful variolation houses. In 1796, his son Daniel Sutton revealed the “secret” techniques. Instead of blood-letting, they administered variolation through just a superficial scratch; in addition, they carefully limited their donors to people suffering only a mild case of smallpox.
Nevertheless, even with these improved techniques, it is estimated that the death rate from variolation was about 2%. Of course, this was a great improvement over the general death rate from smallpox, but it still meant that variolation involved non-negligible risk; hence variolation remained a highly controversial technique.
Variolation was also practiced in the American colonies early in the 18th century. It was largely championed by the Puritan minister Cotton Mather. Apparently Mather received a Libyan-born slave, Onesimus, as a gift in 1706. As described by Arthur Allen in his book Vaccine: “Mather asked him [Onesimus] whether he had had the smallpox. Onesimus showed his master a scar that remained from a childhood variolation in the land of his birth.” Mather subsequently discovered that half a dozen African slaves in the Boston area had also been variolated.
Swayed by this evidence, Mather convinced Dr. Zabdiel Boylston to adopt the practice of variolation. Boylston then variolated 248 patients against smallpox, and in 1726 wrote a book describing the success of his method. In Fig. 1.5 we show the title page of this monograph.
3) Edward Jenner and the Process of Vaccination
By the middle of the 18th century, a fair amount of information had been compiled regarding the viral disease cowpox. Cowpox was a disease that was caused by the cowpox virus, an orthopoxvirus that is related to smallpox. Figure 1.6 shows the forearm of a person who had been infected with cowpox.
Cowpox is transferable from one species to another. Although the disease was known to infect cattle, it is more commonly found in other animals such as rodents. The first humans known to suffer from this disease were dairymaids. When they touched the udders of cows infected with cowpox, they developed the tell-tale pustules (shown in Fig. 1.6) on their hands.
Although cowpox shares many of the characteristics of smallpox, it is a much milder disease than smallpox and is significantly less contagious. It was eventually realized that dairy farmers almost never contracted smallpox. And in 1768, English physician John Fewster pointed out that people who had been infected with cowpox did not seem to contract smallpox. Thus, it was logical to ask whether exposure to cowpox through a procedure analogous to variolation might confer immunity to smallpox.
Edward Jenner (see Fig. 1.7) was a British physician and scientist. When Jenner was a youth, he was successfully inoculated against smallpox. He had apprenticed in surgery at St. George’s Hospital in London before returning to his native Gloucestershire and setting up a medical practice and surgery there.
Upon his return to Gloucestershire in the late 18th century, Jenner became a founding member of The Fleece Medical Society, a group that shared papers on medical topics at The Fleece Inn in Rodborough.
At this time, some of Jenner’s contemporaries were studying whether a vaccine derived from cowpox might serve as an alternative method to protect against smallpox. Between 1770 and 1796, five investigators in England and Germany successfully produced a vaccine from cowpox and tested it on humans.
In 1796, Edward Jenner produced his own vaccine. He scraped pus from the blisters of a dairymaid who had contracted cowpox. He used this material to inoculate an 8-year-old boy, James Phipps, in both arms. Phipps subsequently developed a fever but otherwise showed no signs of infection.
Two months later, after the boy had returned to health, Jenner performed a variolation procedure on Phipps. Under normal circumstances, Phipps would have developed a mild case of smallpox. However, Jenner noted that no infection occurred. Jenner repeated the variolation procedure on Phipps, and once again found no infection. We should mention that by today’s standards, Jenner’s actions with Phipps would be considered morally reprehensible.
Jenner repeated these studies with an additional 23 subjects, and in every case he found that they appeared to possess immunity to smallpox, after being vaccinated with material obtained from someone suffering from cowpox. Jenner submitted a paper to the Royal Society summarizing the results of his study of 24 patients. The Royal Society did not publish his original paper.
In 1798, Jenner published An Inquiry into the Causes and Effects of the Variolae Vacciniae. This was an extremely influential paper. First, it coined the term vaccination, which was derived from the Latin vacca (cow), in recognition of the association with a disease of cattle. Jenner subsequently developed an “arm-to-arm” technique, in which material from one vaccinated individual’s pustule could be injected into another subject.
Jenner’s work had an immediate impact. Following publication of his paper, his technique was widely copied and found to be effective. For example, by 1801 Jenner’s paper had been translated into six languages, and over 100,000 people in Europe had been vaccinated. In 1802, a group of 112 members of London’s Physical Society signed a testimonial to the effectiveness of Jenner’s methods. Figure 1.8 shows a copy of that testimonial.
In his book, The Greatest Killer: Smallpox in History, Donald Hopkins writes: “Jenner’s unique contribution was not that he inoculated a few persons with cowpox, but that he then proved [by subsequent challenges] that they were immune to smallpox. Moreover, he demonstrated that the protective cowpox pus could be effectively inoculated from person to person, not just directly from cattle.”
Jenner’s technique of vaccination against smallpox still had to surmount several challenges. We will review controversies over vaccination in Part II of this series. Suffice it to say that in the 19th century there were active anti-vaccination groups, who crusaded against the practice of vaccination and questioned its benefits.
Here we give a short summary of different objections to vaccination. First, there are objections to vaccination based on religious belief or doctrine. Another class of objections is based on libertarian notions – should individuals be forced to undergo this procedure? A third group of objections one could term “conspiracy theories.” Among these are the claims that the benefits of vaccination are overstated by groups of doctors or companies that manufacture the vaccines, in order to enrich themselves. A different type of conspiracy theory argues that vaccination actually has serious side effects, which are being covered up by the authorities. A recent version of this theory argues that vaccination is a major cause of autism.
Despite the opposition from some quarters, Jenner’s vaccination practices rapidly gained support within the medical community of the time. Because smallpox was such a virulent disease that disfigured or killed so many of its victims, vaccination was seen as a major step forward in public health. Many European countries rapidly adopted the practice and devoted many resources to vaccinating their citizens.
Jenner, known today as the “father of immunology,” was greatly honored during his lifetime. For example, at the time of Jenner’s introduction of vaccination, the French under Napoleon were at war with Britain and other European countries. Napoleon had all of his French troops vaccinated. Jenner made a direct request to Napoleon to release English prisoners of war. Napoleon granted Jenner’s request and permitted English prisoners to return home, saying that he could not “refuse anything to one of the greatest benefactors of mankind.”
4) Development of Vaccination, and Eradication of Smallpox
During the 19th century, cowpox was replaced by vaccinia, a virus that is in the same family as cowpox and smallpox, but is distinct from those two. The smallpox vaccine was a preparation of live vaccinia virus, and was administered by pricking the skin repeatedly with a bifurcated needle that had been dipped into the vaccine solution. A photo of this procedure is shown in Fig. 1.9.
Vaccination by this technique produces a bump that enlarges and then becomes a blister filled with pus. The blister then drains and becomes a scab, which subsequently falls off, leaving a small scar. I have a tiny scar on my left forearm, a result of my childhood smallpox vaccination.
The vaccination procedure was highly effective, although the immunity conferred by vaccination decreased somewhat with time. About 95% of people vaccinated against the disease would not contract smallpox. Of those people who contracted smallpox within 10 years of being vaccinated, the fatality rate from the disease was 1.3%, as compared to a 52% fatality rate among un-vaccinated smallpox sufferers.
There were potential side effects of vaccination. It is estimated that 1 of every 1,000 people who were vaccinated experienced a side effect. This generally involved an allergic reaction to the procedure, but in some cases the vaccinia spread to other parts of the body. More severe reactions were less common – they were estimated at between 14 and 500 per million people. It is estimated that 1 or 2 people per million died from effects associated with the vaccination.
A number of European countries mounted vaccination campaigns against smallpox in their colonies. Arguably the earliest of these efforts was the Balmis Expedition by the Spanish government in 1803. They transported the vaccine to their colonies in the Americas and the Philippines. By 1820 the Dutch had established vaccination programs in their East Indies colonies, and the British had done the same in India and Burma. And in 1832 the United States established a federal vaccination program for Native Americans.
The success of these vaccination efforts led various countries to mount extensive campaigns with the aim of eradicating the disease. In Britain, vaccination became compulsory beginning in 1853. In the United States, Massachusetts became the first state to require mandatory vaccination in 1843, and by 1855 several other states had adopted similar measures.
In developed countries, the incidence of smallpox declined dramatically. By 1900, a number of northern European countries had eliminated smallpox altogether. In 1950, the Pan American Health Organization mounted a campaign to eradicate smallpox in the Western Hemisphere. The initial campaign succeeded in all countries except Argentina, Brazil, Colombia and Ecuador. Then in 1959, the World Health Assembly began a world-wide campaign to eradicate smallpox.
This was a massive undertaking. Despite the progress in developed countries, at this point it was estimated that 2 million people per year were dying from smallpox. Initially this program had considerable success, until the disease was mainly confined to a few underdeveloped countries in unstable conditions. However, final eradication of smallpox required a major effort.
In the last few years of the existence of smallpox, authorities made stringent efforts to identify and wipe out the disease. When smallpox was detected, the sufferers would be isolated and all of the population vaccinated. The last areas where smallpox was detected were Bangladesh and the Horn of Africa. The situation in those areas was complicated by civil war, famine, refugee populations and lack of roads. However, the last known case of Variola major occurred in a 2-year-old Bangladeshi girl in 1975, and the last case of Variola minor was found in a man in Somalia.
The eradication of smallpox was one of the great public-health successes in all of human history. The disease was truly dreadful – the fatality rate from the disease was incredibly high, while survivors were often blind and/or seriously disfigured,. As we have mentioned, in the 20th century alone it is estimated that 300-500 million people died from smallpox.
In 1980, the World Health Assembly issued the following proclamation [World Health Organization (WHO), Resolution WHA33.3]:
“Having considered the development and results of the global program on smallpox eradication initiated by WHO in 1958 and intensified since 1967 … Declares solemnly that the world and its peoples have won freedom from smallpox, which was a most devastating disease sweeping in epidemic form through many countries since earliest time, leaving death, blindness and disfigurement in its wake and which only a decade ago was rampant in Africa, Asia and South America.”
B. Polio, the Polio Vaccine, and the Campaign to Eradicate Polio
The infectious disease Poliomyelitis is commonly referred to as polio. It is also called infantile paralysis, because of the prevalence of the disease among children. It is spread by either contact with infected fecal matter, or with food or water containing infected feces, or less commonly by contact with infected saliva.
The poliovirus is a type of Enterovirus that colonizes the gastrointestinal tract. It causes disease only in humans. Poliovirus is composed of a single RNA genome enclosed in a protein shell called a capsid. Three different serotypes of poliovirus have been identified, and are called PV1, PV2 and PV3, respectively. Figure 1.10 shows a transmission electron microscope (TEM) photograph of the poliovirus.
In roughly 70% of those infected with poliovirus, there are no symptoms. Another 24% of those who suffer polio infection experience minor symptoms such as sore throat and fever. However, in about 1% of infections the virus enters the central nervous system. In the bulk of those cases, the patient develops aseptic meningitis. However, in 1 to 5 cases out of 1000, the polio progresses to a paralytic stage. In these situations the muscles become weak and poorly controlled, and in a few cases this can lead to complete paralysis. In people with paralytic polio, the disease is fatal in about 2-5% of children and 15-20% of adults.
There is no cure for polio. Treatment of non-paralytic cases generally involves providing relief for symptoms and preventing complications. Cases of paralytic polio generally involve more extensive measures such as physical or rehabilitative therapy or even surgery.
For those suffering chest or abdominal weakness, or in cases of quadriplegia, a negative-pressure ventilator called an iron lung was occasionally used. Nowadays polio victims with permanent respiratory paralysis can use jacket-type negative-pressure ventilators worn over the chest and abdomen.
In temperate climates, polio infection was most common in the summer and autumn. In areas with poor sanitation, the most common means of transmission was fecal-oral transmission, either through direct contact with infected fecal material or by ingesting contaminated water or food. In developed countries, poliovirus was most commonly transmitted through the saliva from an infected individual. I vividly remember that in my youth, I was not allowed to swim in public pools in the summer, particularly when any cases of polio had been reported in the region near my home town.
Once an individual has ingested the poliovirus, it binds with an immunoglobulin-like receptor called the poliovirus receptor. The virus begins to replicate and is subsequently absorbed into the bloodstream. Paralytic polio occurs when the virus spreads to nerve fiber pathways, where it destroys motor neurons in either the spinal cord, the brain stem, or the motor cortex.
The likelihood of contracting paralytic polio increases somewhat with age. In children, the most common result is deformation of one limb. Figure 1.11 shows a photo of a girl whose right leg is deformed as a result of contracting polio. In adults, there is a higher chance of paralysis that affects the chest and abdomen, or possibly affecting all four limbs (quadriplegia). The highest rates of paralytic polio occur with the PV1 strain of poliovirus, while the lowest rates are found with the PV2 strain.
It is believed that polio has been present in human populations since prehistoric times. Figure 1.12 shows an Egyptian stele from the 18th dynasty, 1403-1365 BCE. It depicts a person with one thin leg, a deformed foot and a cane; it is surmised that the man had suffered from polio.
In earlier times, poor sanitation conditions meant that the bulk of the population was constantly exposed to poliovirus. Paradoxically, this insured that the population had developed a natural immunity to the disease. When modern sanitation systems were instituted, this meant that fewer people (and particularly fewer children) were exposed to the poliovirus. The net result was an increase in epidemics resulting in paralytic polio.
In Europe and the Americas, epidemics of polio began to appear around 1900. During the first half of the 20th century, polio pandemics appeared in Europe, North America, Australia and New Zealand. The United States experienced its worst polio epidemic in 1952; there were nearly 58,000 total cases of polio reported, resulting in 3,145 deaths and another 21,269 left with some form of paralysis.
The outbreaks of polio gave impetus to concerted efforts to develop a polio vaccine. In 1950, virologist Hilary Koprowski produced a vaccine based on a live but attenuated virus serotype, and administered it to an 8-year-old boy. Her vaccine was used for large-scale trials in the Belgian Congo and was subsequently administered to 7 million Polish children between 1958 and 1960.
Dr. Jonas Salk from the University of Pittsburgh developed a second live, but inactivated polio vaccine in 1952; his breakthrough was announced in 1955. Dr. Salk, bless his soul, did not patent his vaccine but provided it free for others to manufacture and distribute. After 3 doses of the Salk vaccine, 99 percent of recipients developed immunity to poliovirus.
Dr. Albert Sabin subsequently developed a third polio vaccine. Sabin’s oral vaccine was found to produce antibodies to all three strains of poliovirus in 95% of recipients. The U.S. National Institutes of Health (NIH) conducted clinical trials of all existing polio vaccines in 1957, and in 1958 they selected the Sabin vaccine as the standard anti-polio vaccine. It subsequently became the world standard against this disease.
Sabin’s oral polio vaccine had many desirable qualities. It was “inexpensive, easy to administer, and produces excellent immunity in the intestine.” A concerted global campaign against polio was begun in 1988 under the auspices of the World Health Organization, UNICEF, and the Rotary Foundation. This campaign was extremely successful. Figure 1.13 shows the number of worldwide polio cases each year since 1985.
There were 400,000 cases of polio reported in 1985. In 1994, the Americas were reported polio-free, and the last reported case of polio occurred in China. 1998 marked the last case of polio reported in Europe. In 2014, India was certified polio-free. As is shown Fig. 1.14 below, the world is now in what is hoped to be the final stages of the “polio endgame.”
Currently, polio is endemic only in the countries of Afghanistan, Pakistan and Nigeria; and Nigeria has not had a reported case since 2016. In recent years, polio eradication efforts have been slowed by conditions of war and some religious opposition to vaccination. In 2003 a fatwa was issued in Nigeria, claiming that polio vaccine was designed to make children sterile. Polio subsequently re-appeared in that country, but after an organized campaign, no cases have been reported since 2014.
Similarly, the vaccination program in Afghanistan has been hampered by the long-running civil war in that state. In Pakistan, the Taliban has claimed that vaccination is a Western plot to sterilize children. Also, it was rumored that people claiming to be Western doctors were foreign agents involved in the assassination of Osama bin Laden. As a result, several vaccinators were killed. At present, the only two countries currently experiencing endemic cases of polio are Afghanistan and Pakistan.
In 1995, the CDC found that the Sabin live vaccine had a very small chance of producing debilitating side effects, if the virus was not sufficiently weakened. At that point pediatricians in the U.S. switched back to the Salk inactivated-virus form of the vaccine.
In our posts, we stress the fact that side effects of vaccination are generally quite rare. However, when polio vaccines were first marketed in the U.S. in 1955 there was one dramatic and dangerous incident, called the Cutter Incident. Dr. Salk had intended that his vaccine contained killed polio virus. However, Cutter Laboratories shipped batches of vaccine containing live virus.
As described by Dr. Paul Offit in his book Vaccinated: “More than one hundred thousand children were inadvertently injected with live, dangerous polio virus. Worse, children injected with Cutter’s vaccine spread polio to others, starting the first and only man-made polio epidemic. When the dust settled, live polio virus contained in Cutter’s vaccine had infected two hundred thousand people; caused about seventy thousand to have mild cases of polio; permanently and severely paralyzed two hundred people, mostly children; and killed ten.”
Another side effect issue arises with the oral form of vaccine, which can occasionally mutate to produce chronic infections in vaccinated individuals. Those individuals can then transmit the mutated virus to others. This is currently happening in Syria, and the Congo. Efforts are being made to replace the oral vaccine by an inactivated vaccine such as Salk’s that cannot mutate. At present, these circulating vaccine-derived polio infections outnumber the “wild” cases around the world. It is hoped that in a few years, polio will join smallpox as the second major human disease to be eradicated from the globe.
C. Herd Immunity
In the 1930s, it was noted that following a campaign to vaccinate children against measles, the incidence of measles in un-vaccinated children decreased. This was attributed to a condition called herd immunity. The cartoon in Fig. 1.15 demonstrates how herd immunity works.
In the cartoon above, when none of the population is immune to a particular infectious disease, the presence of a few sick people will cause the disease to spread through the population. Once some of the population are immune to the disease, a contagious disease will still spread but more slowly. However, when the majority of the population are immune to the disease, a small number of sick and contagious individuals will not be able to spread the disease. The disease will be contained, and epidemics will not occur.
Herd immunity can occur in populations where individuals develop immunity naturally through exposure to certain infectious diseases. However, once the mechanism of herd immunity was understood, it was seen as an important element of mass vaccination campaigns. The goal was then to insure that a sufficiently high percentage of the at-risk population was immunized against a particular disease.
The concept of herd immunity gave rise to the practice of ring immunity, that is widely used in modern campaigns to eradicate contagious diseases. In a disease-eradication campaign, health workers will vaccinate everyone who has close contact with a person suffering from the disease (they will vaccinate in a ring around the infected individual). This will help slow the spread of the disease.
One can derive a mathematical formula that expresses the mechanism of herd immunity. Assume that a population is completely susceptible to a given disease, and further assume that the population is homogenous (i.e., that each individual has the potential to come into contact with every other susceptible person). Let R0 be the average number of new infections caused by each infected individual in a non-immunized population. Then
R0(1-p)=1, (1)where p is the proportion of the population that attains immunity. This equation can be solved to find the critical proportion of the population pc that needs to be immunized in order to stop the spread of the disease,
pc=1-1/R0. (2)The quantity pc is also called the herd immunity threshold or HIT. The quantity R0 depends on the method of transmission of the disease, and how easily it is acquired. Both smallpox and polio have R0 in the range 5-7, which leads to an HIT of 80 -86%. The more contagious diseases measles and pertussis have R0 in the range 12-18, which leads to an HIT of 92 -94%. On the other hand, for the flu, which is less contagious, herd immunity is achieved when 50-75% of the community are immunized, and the Ebola virus has R0 in the range 1.5-2.5, which leads to an HIT of 33- 60%.
Now, as more of the population develops immunity, the quantity R0 decreases to some effective transmission rate Re. As can be seen from Eq. (2), once Re decreases below 1, then the number of new cases will diminish until the disease is eliminated. Now, the assumption of homogeneity of the population is not accurate. However, Eqs. (1) and (2) are still valid if we replace R0 by an effective infection rate.
The most effective way to boost immunity in a population is through mass vaccination. Natural infections can also confer immunity, but vaccination does not produce the adverse side effects that arise from suffering the disease. Note that herd immunity means that one does not have to vaccinate every member of a community in order to eradicate a disease, provided that a greater percentage of people are vaccinated than the HIT. This means that the benefits of a vaccination campaign can be greater than the fraction of people who are vaccinated.
The herd immunity concept does have one drawback. If a disease mutates to a new form that is not blocked by the existing vaccine, then the population may be susceptible to this new form of the disease.
Note that herd immunity is vulnerable to what is called the free rider problem, that is, individuals who choose not to vaccinate. In any vaccination campaign there will be those who have medical conditions such that vaccination is contra-indicated. Free riders include this group, in addition to those who could be vaccinated but who voluntarily opt out. If the number of free riders is sufficiently small, this does not cause a problem because the fraction of vaccinated people is still greater than the HIT.
However, when the fraction of free riders is larger than the quantity (1-HIT), the population becomes more and more susceptible to pandemics of an otherwise preventable disease. As we will see in the next section, some parts of the U.S. have recently seen a significant decline in vaccinations, particularly for measles. In the past few years, outbreaks of measles have occurred that appear to be a direct result of declines in the number of vaccinated children.
To see an example of herd immunity (or lack thereof) in action, Fig. 1.16 shows a graph of MMR (measles-mumps-rubella) vaccination rates for children in the state of California. An inset shows the location of measles cases in that state over the years 2014-2015. We previously stated that the HIT rate for measles was in the range 92-94%. Note the strong correlation between those counties where the vaccination rate fell below 95%, and areas where measles outbreaks occurred. This is exactly the outcome predicted by the notion of herd immunity.
D. Vaccination Campaigns for Other Diseases
We have focused here on smallpox and polio because these are the diseases that have either been eradicated or nearly eradicated from the globe. However, vaccines have been developed for several other diseases. A 2011 article in the Journal of the American Medical Association (JAMA 306, 36 (2011)) quoted an economic analysis of the effects of childhood vaccination. The article estimated that at 2010 rates, immunization prevented about 42,000 deaths per year and 20 million cases of disease. The net annual savings were $14 billion in direct costs, and $69 billion in total societal costs. In this section, we will provide short reports on the contagious diseases measles and mumps.
Measles is an extremely contagious infectious disease that is caused by the measles virus. Initial symptoms include fever, cough and inflamed eyes. This is followed by a rash that generally starts on the face and then spreads to other parts of the body. Figure 1.17 shows a child with a measles rash.
Measles is an airborne disease that can be spread by coughs, sneezes or saliva of infected persons. Nine out of ten people without immunity to the disease will catch it if they share space with an infected individual.
In developed countries, a disease such as measles often results in fairly minor symptoms. In countries with poor sanitation conditions and relatively low vaccination rates, measles can lead to bronchitis, pneumonia, brain inflammation and encephalitis. Worldwide rates of death from measles have progressively decreased due to vaccination campaigns, as shown in Fig. 1.18. WHO estimated that in 2016 there were 89,780 deaths worldwide from measles. This is an all-time low, and is a clear sign that the massive vaccination campaign is working.
A measles vaccine was developed in the mid-60s by Maurice Hilleman, and in the late 60s improved measles vaccines were introduced. Nowadays the most common vaccine is a combination measles/mumps/rubella (MMR) vaccine, or more recently MMRV (which adds a vaccine for varicella or chickenpox). It is recommended that it be given to children at age 12 months, a time when their immune systems are sufficiently developed.
Public health campaigns with the MMR vaccine have been carried out worldwide. In the U.S., endemic or “wild” cases of measles were eradicated by 2000, and endemic cases of measles have been eradicated in the Americas since 2016. Vaccination campaigns in the rest of the world have continued, through the joint efforts of the Measles Initiative, the Red Cross, the Centers for Disease Control, the United Nations Foundation, UNICEF, and WHO. Today, the vast majority of measles deaths occur in Southeast Asia and Africa.
An outbreak of measles in Ohio in 2014 occurred after two unvaccinated Amish men returned from a missionary trip to the Philippines. Their Amish community had relatively low vaccination rates, which caused an outbreak that eventually included 383 cases of measles, 89% of which occurred in unvaccinated individuals. In Dec. 2014 a measles epidemic occurred in California. The outbreak appears to have originated at the Disneyland theme park. Its origin was never determined, and it eventually led to 147 cases of measles.
It should be emphasized that the highly contagious nature of measles can allow it to spread extremely rapidly through unvaccinated populations. Fighting epidemics of measles is rather costly – it is estimated that it cost $1 million to fight the 2014 Ohio measles outbreak. Such outbreaks put unvaccinated individuals and children at particular risk, as well as those with compromised immune systems.
Mumps is an infectious disease caused by the mumps virus. Up to 40 percent of mumps cases are asymptomatic, so a fairly large number of infected people may not realize that they are carrying the disease. Initial symptoms of mumps include fever, muscle pain and lethargy. This is followed by a painful swelling of one or both parotid salivary glands. Mumps is generally more severe in adults than in children. Complications of mumps include meningitis, pancreatitis, deafness and inflammation of the testicles, which can lead to sterility.
Mumps is also an airborne disease, spread by respiratory droplets. Two rounds of vaccination can provide immunity against mumps, and a mumps vaccine was developed by Maurice Hilleman of Merck in the mid-60s. Today, vaccination is provided by the MMR vaccine. In the U.S. this is generally administered to children at 12-16 months of age, and then repeated to preschool children 3-5 years of age. The vaccine is effective against the disease in roughly 80% of those vaccinated.
In the U.S. there were 151,000 cases of mumps reported in 1968. In 2006, there was a mumps outbreak that spread widely in college campuses, particularly in the Midwest, and may have involved up to 6,000 cases. Apart from that epidemic, from 2001 to 2008 there averaged 265 cases of mumps per year in the U.S.
—- Continue to Part II —
In preparing the three parts of this review, we have made significant use of the following sources:
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