Polio vaccine

The polio vaccine emerged from a scientific competition that played out over two decades, with two fundamentally different approaches—killed virus versus live attenuated virus—each representing a different bet on immunology. The adjacent possible opened in the 1930s with improved tissue culture techniques, but the path to a working vaccine required solving a cascade of problems: growing poliovirus in quantity, inactivating it reliably, and proving safety in millions of children.

Poliomyelitis terrorized mid-century America unlike any other disease. The virus struck unpredictably, paralyzing children and filling hospital wards with iron lungs. President Franklin Roosevelt, paralyzed by polio in 1921, founded the National Foundation for Infantile Paralysis—the March of Dimes—which would fund the vaccine development with unprecedented public donations.

Jonas Salk, a researcher at the University of Pittsburgh, chose the killed-virus approach. By 1952-1953, he had developed an inactivated polio vaccine using formaldehyde to kill wild polioviruses of all three strains while preserving their ability to trigger antibody production. Salk tested the vaccine on himself and his family in 1953—the same self-experimentation tradition stretching back to Edward Jenner. In 1954, 1.6 million children in the United States, Canada, and Finland participated in the largest clinical trial in history.

On April 12, 1955—ten years to the day after Roosevelt's death—the results were announced: the vaccine was safe and effective. Salk became an instant hero. When asked who owned the patent, he replied: "The people, I would say. There is no patent. Could you patent the sun?" He had declined to patent the vaccine, sacrificing millions in potential royalties to ensure the widest possible distribution.

But triumph turned to tragedy within weeks. Two batches produced by Cutter Laboratories in Berkeley contained live poliovirus that the formaldehyde had failed to inactivate. The Cutter Incident caused 192 cases of paralytic polio and 11 deaths among vaccinated children and their families. The program was suspended. Federal vaccine production requirements were rewritten. Public trust in vaccines suffered damage that would echo for decades.

Meanwhile, Albert Sabin had been developing a different approach: a live attenuated vaccine using weakened virus strains that could be administered orally rather than by injection. Sabin tested his vaccine on himself, his family, and prison volunteers, but with Salk's vaccine already in use, he could not conduct large-scale trials in the United States. Instead, the Soviet Union became his testing ground—between 1955 and 1961, over 100 million people in the USSR received Sabin's oral vaccine.

Sabin's vaccine entered commercial use in 1961 and eventually replaced Salk's in most of the world. The oral vaccine was cheaper, easier to administer, and provided intestinal immunity that could prevent transmission. Like Salk, Sabin refused to patent his invention. Together, the two vaccines reduced annual polio cases worldwide from an estimated 350,000 in 1988 to 33 in 2018—the closest humanity has come to eradicating a disease since smallpox.

The convergent development—two researchers independently solving the same problem through different mechanisms—demonstrates that the adjacent possible had opened wide enough for multiple solutions. The tissue culture techniques, the virus typing, and the immunological understanding all aligned in the early 1950s. Whether killed or live, the polio vaccine was coming.