In the West, it feels like a disease from another age. But on July 21, the United States reported its first case of polio in nearly a decade – a 20-year-old man from Rockland County, 30 miles from New York City. He went to the hospital for leg paralysis and was quickly diagnosed with poliovirus, the disease's causative agent, and is still partially paralyzed. He had not been vaccinated against the disease.
Wealthy countries had almost forgotten about this highly contagious disease, caused by a virus that attacks the nervous system and can cause permanent motor disability. It is usually transmitted through the fecal-oral route, via contaminated water or food. Before the start of mass vaccination campaigns in the mid to late-1950s in the West, the disease was a frightening specter – especially for parents with children under five, whom the virus targets in particular. In France, between 1943 and 1988, polio caused 32,793 cases of paralysis and 3,315 deaths.
Since 1988, the number of polio cases has dropped by 99% – the result of the Global Polio Eradication Initiative spearheaded by national governments, the World Health Organization, Rotary International, the US Centers for Disease Control and Prevention, and UNICEF. It also received massive support from the Bill and Melinda Gates Foundation and Gavi, the Vaccine Alliance.
In 1988, the global number of cases was estimated to be 350,000, with the virus endemic in more than 125 countries. By 2021, only six cases of paralysis due to wild poliovirus were reported and only two countries, Afghanistan and Pakistan, continued to experience wild virus-related polio outbreaks.
The young American from Rockland County, however, was not infected in either of these countries. His paralysis was the result of a disturbing phenomenon that has been observed elsewhere – despite the fact that he had not been vaccinated, the virus that affected him originated from an oral vaccine used in many developing countries. The vaccine contained a poliovirus that had been rendered harmless but was still live, and then mutated because it multiplied in multiple unvaccinated people.
The following scenario played out: a person receives one of these oral vaccines and they excrete the poliovirus in their stool for several weeks. "If there are only orally vaccinated people around, this virus will not find anyone to infect," said Maël Bessaud, a poliovirus expert at the Pasteur Institute. "But if the virus finds enough unvaccinated people, or if it ends up in an area where only the inactivated vaccine is used, this virus may start to circulate." If it spends several months spreading from one unvaccinated person to another, it can modify its genome, become virulent again and cause paralysis.
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