On April 12, 1955, virologists announced that they had developed a safe and potent vaccine against poliothe deadly, paralyzing disease which then tortured thousands of people in the US Launching a series of concentrated vaccination campaigns at home and abroad, the announcement sowed the seeds for a polio-free future.
Today, 68 years after the arrival of the vaccine, the disease teeters on the brink of extinction. IN USA, cases of wild poliomyelitis circulating in the community were exterminated in the 1970s and have since been driven from almost all corners of the world.
So, in honor of its announcement in April, here’s what you’ll want to know about the history of the polio vaccine.
The polio plague
Although the virus that causes polio has been circulating everywhere human history, it was not until 1900 that the poliovirus caused seasonal waves of infection. The first of these outbreaks to occur in the US occurred in Vermont in 1890, infecting 132, paralyzing 58 and killing 18. Only waves of worse outbreaks followed, troubling the population (and especially the young) for the next 60 years or so such.
The reason for the sudden onset of these seasonal, summer outbreaks remains a mystery. Some experts see the creation and spread of stronger strains of the poliovirus as the culprit. Others credit improving sanitation systems in the US with preventing people from developing immunity to the virus in infancy.
However, whatever the cause, the sudden onset of an annual polio season was devastating. In 1952, the worst wave of the disease infected 60,000 people in the U.S., many of whom were left with canes, crutches, wheelchairs and full-body ventilators, dubbed “iron lungs” after the wave.
With polio ravaging the population and no cure in sight, a panel of experts supported by the National Polio Foundation set out to fight the virus with vaccines. At the time, the idea of fending off the virus with vaccines was nothing new. But producing a polio vaccine was still a difficult task, requiring a thorough knowledge of the transmission of the virus.
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The pathology of poliomyelitis
Poliomyelitis or poliomyelitis is an intestinal infection caused by the poliovirus, which is passed from person to person by ingesting the feces of an infected individual.
Infections are mostly mild, including symptoms such as fever, fatigue, and nausea. But in about 1 in 200 infections, the virus travels to the nervous system, where it destroys the neurons that direct muscle movement. When this happens, the virus can cause permanent paralysis and sometimes death when the muscles in the individual’s airways are adversely affected.
In the 1940s, researchers learned a lot about poliovirus. In 1949, a team discovered that there were three types of viruses in circulation, including PV1, PV2, and PV3, which needed to be targeted and treated with a vaccine on an individual basis. That same year, another team discovered that these three virus types could be cultured artificially, which would provide a safe and consistent flow of poliovirus from which a potential vaccine could be created.
Read more: The deadly polio epidemic and why it matters for the coronavirus
Salk’s polio vaccine
Armed with the knowledge of the three types of poliovirusas well as a steady supply of all three, the American virologist Jonas Salk developed the first promising polio vaccine in 1952. After several small trials, the vaccine was tested in the Francis Field Experiment, then the largest field medical test in the United States
Named for Thomas Francis, the trial’s administrator, the trial involved nearly 2 million individuals who received injections of the vaccine or a placebo, or who received nothing. After observing participants to determine whether the vaccine reduced their chances of contracting polio, Francis concluded in April 1955 that the vaccine was 60 to 70 percent effective against PV1 and 90 percent effective against PV2 and PV3.
In a departure from the traditional vaccine development process, Salk’s vaccine did not contain any active strains of the virus it was intended to defeat. Instead, it contained only inactivated strains of the virus, incapacitated by formaldehyde, which could induce antibodies without inducing infection. As one of the early pioneers of this type of vaccine, Salk was convinced that these strains were as potent as the active strains but much safer when included in vaccinations.
The trial results inspired immediate approval of the vaccine. Soon, millions of vaccinations were shipped across the United States, where they were distributed primarily to “polio pioneers” who participated in trials without receiving doses of the drug themselves. After this initial wave of vaccinations, infections declined and deaths dropped by 50 percent after one year.
Sabin’s polio vaccine
Within two years of the announcement, annual polio infections dropped to about 5,600 in the United States. Within six years of the announcement, infections fell to 160. This further drop in infections was due to the development of an alternative, active version of the vaccine, developed by virologist Albert Sabin and made available to the public in 1961.
While Sabin admits that the inactive strains included in Salk’s vaccine can temporarily induce antibodies, the virologist believes that an active virus is needed to create antibodies that really stick around. As such, Seibin is examining a range of active poliovirus variants, looking for strains that are too weak to cause serious symptoms but strong enough to elicit a lasting antibody response.
By 1961, Sabin had discovered three of these attenuated strains and incorporated them into an active polio vaccine. Administered orally by dripping on the tongue or infused with sugar, Sabin’s active vaccine is easy and affordable to administer. Once approved, it immediately displaced Salk’s injected vaccine to become the most popular method of protection against polio in the US
Polio vaccines in the United States
Throughout the virologist’s career, Sabin insisted that the chances of contracting paralytic polio from an active vaccine were minimal. But despite Sabin’s ideas, nearly 110 cases of vaccine-associated polio occurred in the US between 1980 and 1992, when the weakened virus included in the vaccine regained its virulence after ingestion.
In response to these infections, the US reverted to the inactive vaccine in 2000. Since then, all vaccines administered against polio in the country were inactive, completely eliminating the risk of contracting polio from the vaccine.
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