On April 12, 1955, virologists announced that they’d developed a safe and potent vaccine against polio, the deadly, paralyzing disease that then tormented thousands of individuals in the U.S. Setting off a series of concentrated vaccination campaigns in the country 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 verge of disappearance. In the U.S., cases of wild, community-circulating polio were wiped out in the 1970s, and since then, they’ve been banished from almost all corners of the world.

So, in honor of its April announcement, here’s what you’ll want to know about the history of the polio vaccine.

The Plague of Polio

Though the virus that causes polio has circulated throughout human history, it wasn’t until the 1900s that the poliovirus prompted seasonal spates of infections. The first of these outbreaks to occur in the U.S. took place in Vermont in the 1890s, infecting 132, paralyzing 58, and killing 18. And waves of worse outbreaks only followed, troubling the population (and particularly the young) for the next 60 years or so.

The reason for the sudden onset of these seasonal, summer outbreaks remains a mystery. Some specialists see the creation and circulation of stronger strains of the poliovirus as the culprit. Others credit improving sanitation systems in the U.S., which prevented people from increasing their immunity to the virus in their infancy.

That said, regardless of the reason, the sudden appearance of an annual polio season was devastating. In 1952, the single worst wave of the disease infected 60,000 individuals in the U.S., many of whom were left with canes, crutches, wheelchairs and full-body breathing apparatuses called “iron lungs” in the aftermath of the surge.

With polio ravaging the population and no cure in sight, an assembly of specialists supported by the National Foundation for Infantile Paralysis set out to fight the virus with vaccines. By that time, the notion of fending off a virus with vaccines was nothing new. But producing a vaccine against polio was still a tricky task, requiring an intimate knowledge of the transmission of the virus.

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The Pathology of Polio

Polio, or poliomyelitis, is an intestinal infection caused by the poliovirus, which passes from person-to-person through the ingestion of the feces of an infected individual.

The infections are mostly mild, involving symptoms such as fever, fatigue and nausea. But in around 1 in 200 infections, the virus travels to the nervous system, where it destroys the neurons that direct muscle movement. When this occurs, the virus can cause permanent paralysis and sometimes death, whenever the muscles within an individual’s airways are adversely affected.

Throughout the 1940s, researchers learned a lot about the poliovirus. In 1949, a team discovered that there were three types of virus in circulation, including PV1, PV2 and PV3, which would need to be targeted and treated by a vaccine on an individual basis. That same year, another team discovered that these three types of virus could all be cultivated artificially, which would supply a stream of poliovirus, safely and consistently, from which a potential vaccination could be created.

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Salk’s Polio Vaccine

Armed with the knowledge of the three types of poliovirus, as well as a steady supply of all three, American virologist Jonas Salk developed the first promising vaccine against polio in 1952. After several small trials, the vaccine was tested in the Francis Field Trial, which was then the most massive medical field test in the U.S.

Named after Thomas Francis, the administrator of the test, the trial involved almost 2 million individuals who received injections of a vaccine or a placebo, or who received nothing at all. After observing the 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.

Breaking from the traditional process of vaccine development, Salk’s vaccine did not contain any active strains of the virus that it 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 first pioneers of this type of vaccine, Salk was convinced that these strains were as potent as active strains, but much more secure when included in vaccinations.

The results of the trial inspired the immediate approval of the vaccine. Soon, millions of vaccinations were transported throughout the U.S., where they were distributed, first and foremost, to the “polio pioneers” who participated in trials without receiving doses of the drug themselves. Following this initial wave of vaccinations, infections dipped, and deaths decreased by 50 percent after a single year.

Sabin’s Polio Vaccine

Within two years of the announcement, annual infections from polio plummeted to around 5,600 in the U.S. Within six years of the announcement, infections plunged to 160. This additional 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 acknowledged that the inactive strains included in Salk’s vaccine could induce antibodies temporarily, the virologist thought that an active virus was required to raise antibodies that really stuck around. As such, Sabin studied an assortment of active poliovirus variants, searching for strains which were too weak to cause serious symptoms but were strong enough to evoke an enduring antibody response.

By 1961, Sabin found three of these attenuated strains, and incorporated them in an active vaccine for polio. Administered orally, by being dropped onto the tongue or infused in sugar, Sabin’s active vaccine was simple and affordable to administer. Once approved, it immediately supplanted Salk’s injected vaccine, becoming the most popular method of protection against polio in the U.S.

Polio Vaccines in the U.S.

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, almost 110 cases of vaccine-associated polio arose in the U.S. between 1980 and 1992, occurring when the weakened virus incorporated in the vaccine regained its virulence after ingestion.

In response to these infections, the U.S. returned to the inactive vaccine in 2000. Since then, all vaccines administered against polio in the country have been inactive, completely removing the risk of contracting vaccine-associated polio.

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