On April 12, 1955, every American newspaper and TV set jubilantly announced that Jonas Salk’s polio vaccine was a success. Just three years earlier, during the worst polio outbreak in U.S. history, 57,000 people were infected, 21,000 were paralyzed and 3,145 died, most of them children. Pools and movie theaters were shuttered, and panicked parents kept their kids at home, haunted by black-and-white images of toddlers in leg braces and rows of infants sealed in iron lungs.

Nationwide, news of the Salk vaccine was greeted with tears of joy and relief. Even the usually stoic President Dwight D. Eisenhower’s voice broke when expressing his gratitude to Salk in a Rose Garden ceremony, writes historian David Oshinsky in his Pulitzer Prize-winning 2005 book, Polio: An American Story. “I have no words to thank you,” said Eisenhower, himself a father and grandfather. “I am very, very happy.”

But as Oshinsky explains, that sense of deliverance felt in April 1955 soon gave way to frustration and doubt as the rollout of the Salk vaccine faced critical shortages, a deadly contamination crisis and competition from a rival polio vaccine.

In the early 1950s, the U.S. government played a limited role in public health. In fact, the campaign to cure polio was mostly funded by charitable donations to the National Foundation for Infantile Paralysis (now the March of Dimes), which awarded generous grants to vaccine researchers like Salk at the University of Pittsburgh.

The American public was deeply invested in fighting polio, with 300,000 volunteers from all walks of life helping to complete the Salk vaccine trial in 1954, a massive and unprecedented undertaking. At over 200 test sites nationwide, volunteers inoculated nearly 2 million children, some with the real vaccine and others with a placebo as part of the first double-blind vaccine trial in American history.

“The whole nation was united behind this search for a vaccine,” says Dr. Rahul Gupta, chief medical and health officer at the March of Dimes. “It had as much to do with community alignment and engagement as it did with science.”

When the trial was deemed a success, the public expected that the government had been slowly stockpiling enough Salk vaccine to immunize every infant, child and young person in America. But that simply wasn’t the case. Oshinsky writes that the Eisenhower administration and its Secretary of Health, Education and Welfare (HEW) saw vaccine production and distribution as the responsibility of private pharmaceutical companies. When the Salk vaccine was approved, the federal government didn’t have a single injection available.

Instead, the initial rollout of Salk’s miracle vaccine was handled again by the National Foundation for Infantile Paralysis, which had procured nine million shots, not nearly enough for widespread immunization, but enough to target the youngest and most vulnerable kids, with careful rationing. Most other Americans under 18 would have to wait through another anxious summer of pool closures and social distancing.

Asked by a Senate committee why the Eisenhower administration wasn’t better prepared for nationwide vaccine distribution, the HEW Secretary Oveta Culp Hobby responded, “I would assume that this is an incident unique in medical history,” reports Oshinsky. “I think no one could have foreseen the public demand.”

Canada had, apparently. That country’s Ministry of Health, with the full support of Canadians, had immediately taken over polio vaccine production and distribution, resulting in a plentiful supply of Salk vaccine and a nationwide inoculation campaign for all youth.

Six pharmaceutical companies were licensed in 1955 to produce the Salk vaccine, which was a type of “killed” vaccine. All vaccines work by introducing a virus into the body and training the immune system to produce targeted antibodies for that disease. The Salk vaccine was a series of injections with a dead virus that would produce polio antibodies in the bloodstream.

The virus in the Salk vaccine was killed by exposure to formaldehyde, or at least that’s what was supposed to happen. But on April 24, 1955, just weeks after the first children were vaccinated, a doctor in Pocatello, Idaho, reported that a 7-year-old patient named Susan Pierce was suffering from fever and paralysis in her left arm, the same arm that was injected with the Salk vaccine. Three days later, little Susan was dead.

Tragically, Susan wasn’t alone. In Idaho alone, 14 more polio cases were reported that same week, more cases than the state usually experienced in an entire Spring, reports Oshinsky. Even more alarming, a 33-year-old Idaho mother died from polio after apparently contracting it from her vaccinated children.

After hundreds more cases of vaccine-related infections were reported nationwide, the surgeon general halted all vaccinations on May 8, 1955 until a cause could be determined. An emergency investigation traced all of the infections to one batch of Salk vaccine manufactured by Cutter Laboratories in California. Proteins in the vaccine had clumped together, preventing the formaldehyde from fully killing the virus. Not only had children been injected with live polio virus, but they had spread their active infection to family and friends.

Nationwide, hundreds of children and adults were paralyzed with polio and 11 people died from the vaccine tragedy known as the “Cutter incident,” which undermined the public’s faith in Salk’s miracle vaccine. The New York Times ran a story on May 8, 1955 describing how the original “wave of exuberance” over the vaccine was being replaced in less than a month with “confusion, conflict, and doubt.”

Once the source of the polio infections was discovered, vaccinations were allowed to continue, but the Cutter incident stained the integrity of the Salk vaccine and opened the door for a competing polio cure developed by Salk’s scientific rival, Albert Sabin, director of Cincinnati’s Children’s Hospital.

Unlike Salk’s killed virus, Sabin’s vaccine was made from a live “attenuated” virus, meaning a weakened virus that’s strong enough to produce antibodies, but too weak to cause an active infection. Also, the Sabin vaccine was taken orally in one dose as opposed to receiving multiple injections of the Salk vaccine. The oral vaccination route had distinct advantages, explains Gupta from the March of Dimes.

“Al Sabin said, ‘The way this virus infects is through the GI tract and the way we have to fight this is through the GI tract,’” says Gupta. “He was working from the inside out. Also, a live attenuated virus would actually shed through fecal contamination and provide herd immunity.”

It was the Soviet Union, America’s Cold War enemy, that was the first to test the Sabin vaccine. Sabin was born in Poland, then a satellite state of the Soviet Union, and accepted the communist nation’s invitation in 1959 to conduct a massive trial of his oral vaccine on 10 million Soviet children. When the trial was a success, the Soviets immediately ordered the vaccination of every person under the age of 20 with Sabin’s vaccine, a total of 77 million people, writes Oshinsky.

Gupta says that the Soviet investment in the oral vaccine made Sabin’s polio cure available across Eastern European countries like Lithuania and Estonia by 1960. Back in the United States, widespread inoculation with the Salk vaccine had brought new polio infections down from more than 30,000 in 1955 to just 1,000 in 1961.

Even with that incredible success rate, writes Oshinsky, the Salk vaccine was losing favor. American politicians wanted to know why Soviet children were being treated with an American scientist’s vaccine, and whether the United States was losing ground to its enemy—a “vaccine gap” akin to the missile gap. U.S. pediatricians petitioned the American Medical Association for guidance, and in 1961 the AMA handed down its recommendation that the Salk vaccine be replaced with Sabin’s oral formula.

From the early 1960s onward, the global fight against polio was largely accomplished by the Sabin vaccine, not Salk’s. In 1962, for example, Cuba launched its annual vaccination campaign to inoculate all of its children from ages 1 month through 14 years with the Sabin vaccine. Thanks to the campaign, there were only 10 confirmed cases of polio in Cuba from 1963 to 1989 and the World Health Organization declared Cuba polio-free in 1994.

During the 1960s and 1970s, Oshinsky writes that polio vaccination was commonplace in large, developed nations worldwide, including Australia, China, Japan, most of Europe and large portions of Central and South America. Almost all of these global vaccination programs used the Sabin vaccine with the exception of Scandinavia, whose government-run health systems stuck with Salk’s formulation. The widespread adoption and success of the oral vaccine prompted Sabin to boast in 1985 that his creation had “probably prevented about five million cases of paralytic poliomyelitis during the past 20,” reports Oshinsky.

At the same time, the WHO identified pockets of the globe where the wild polio virus still ran largely unchecked. In 1987, the health organization launched its Global Polio Eradication Initiative (GPEI) to target endemic polio in 22 countries. And in the year 2000, the Bill and Melinda Gates Foundation partnered with other nonprofits to create the Global Alliance for Vaccines and Immunization or Gavi, which has also invested billions in childhood vaccinations against polio and other deadly viruses to safeguard the world’s neediest children.

When Gupta was in medical school in India, he participated in New Dehli’s first polio campaign in 1994, which diverted all of the city’s healthcare resources to the cause. “We vaccinated 2.5 million kids in one day,” he says. “We did nothing but vaccinate for polio.”

Today, thanks to both the Salk and Sabin vaccines, infections from the wild-type poliovirus have been eradicated worldwide in all but two countries, Pakistan and Afghanistan, which reported 176 new infections in 2019.