Oral polio vaccine

When Jonas Salk announced his inactivated polio vaccine in 1955, he solved one problem but left another: polio is an enteric virus living in the intestine, spreading through fecal contamination. Salk's injected vaccine produced systemic immunity but limited intestinal immunity. Vaccinated individuals could still harbor the virus in their gut and shed it to others.

Albert Sabin, working at Cincinnati Children's Research Foundation, had demonstrated in 1939 that polio spreads through fecal-oral routes. He isolated three attenuated strains by 1957 but faced a paradox: with Salk's vaccine already deployed in America, he couldn't secure domestic trials. He turned to the Soviet Union, which conducted massive field trials—10 million in 1959, over 70 million by 1960.

The results demonstrated three advantages. First: ease of administration—no needles, no sterile equipment. Hungary and Czechoslovakia achieved rapid coverage through mass campaigns. Czechoslovakia became the first country to eliminate polio. Second: superior immunity—the live virus replicated in the intestine, triggering both systemic antibodies and local secretory IgA, blocking viral replication. Even more remarkably, excreted vaccine virus could passively immunize unvaccinated contacts. Third: cost—at $0.11-$0.20 per dose versus IPV's higher price, economics mattered for billions of children.

The U.S. approved Sabin's vaccine in 1961. By mid-1960s, OPV had largely replaced IPV and became the cornerstone of global eradication. The Global Polio Eradication Initiative, launched 1988, reduced cases by 99.9%—from 350,000 annually to 99 in 2024. Wild types 2 and 3 were declared eradicated.

But the attenuated vaccine retained one dangerous property: it could mutate back to neurovirulence. Vaccine-derived polioviruses cause outbreaks in areas with low coverage. As eradication approaches, the tool that brought humanity to the threshold must itself be eradicated. Sabin refused to patent his vaccine, ensuring low cost and wide distribution. That choice enabled billions of vaccinations.