The global pandemic isn’t the only current example of what happens when vaccine skepticism and misinformation infect the public. Childhood vaccination rates around the world are also experiencing their largest drop in three decades. And on July 21, another viral disease with remarkably effective vaccines to prevent it made a very unwelcome appearance. A single case of polio turned up in Rockland County, New York—the first time any case of polio had been detected in the U.S. in nearly a decade. The infected individual, whose name is being withheld, is a man in his twenties who recently developed polio-related paralysis. He had never been vaccinated against the disease.
A single case in a single county is a bigger deal that it may seem. Polio was declared officially eradicated in the U.S. in 1979, and there are only two countries left in the world in which the disease is endemic: Pakistan and Afghanistan. The fact that even a sole case of polio has made it to the U.S. is troubling.
“Many of you may be too young to remember polio, but when I was growing up, this disease struck fear in families, including my own,” said Rockland County Executive Ed Day in a statement to the media on July 21. “The fact that it is still around decades after the vaccine was created shows you just how relentless it is.”
But though the case is worrisome, its epidemiological route is not a mystery, and a polio vaccine paradoxically played a role in the transmission. There are two kinds of polio vaccine: the inactivated polio vaccine (IPV) and the oral polio vaccine (OPV). The IPV, as its name suggests, uses a killed poliovirus to sensitize the body’s immune system to the disease and prepare it to defend itself against a wild virus, should one be encountered later. The OPV uses an attenuated—or weakened—form of the virus to perform the same function. The big advantage of the OPV is that it can be administered easily and cheaply—simply with a few drops on the tongue—by nearly any health-care worker. The IPV must be injected.
Ease of administration is the reason the OPV has historically been the vaccine of choice for global polio-eradication programs, but it has a drawback. On exceedingly rare occasions, the attenuated vaccine can mutate back to virulence. Sometimes this can cause polio in the vaccinated individual; other times, that person is unharmed but can still shed the virus, causing polio in an unvaccinated person who encounters it. The OPV prevents vastly more cases of disease than it causes. The Global Polio Eradication Initiative (GPEI) estimates that the 10 billion doses of OPV administered worldwide in the past decade have prevented more than 16 million cases of polio, compared to the tiny handful they have caused. Still, the fact that the vaccine presents any such risk at all was behind a decision to discontinue its use in the U.S. in 2000 and administer the IPV only.
In the case of the New York patient, genetic testing of the virus he contracted indicated that it was vaccine-derived. The New York man had not traveled out of the country recently, so epidemiologists suspect that the virus was carried in by someone from overseas who had received the OPV.
The answer for all Americans is to be up to date on vaccinations and to get children vaccinated as well. According to the U.S. Centers for Disease Control and Prevention, 92.6% of two-year-old children in the U.S. have received their recommended three IPV shots. That’s a lot, but someone in the remaining 7.4% of unvaccinated children could be the next case, like that of the young man in New York. That’s a fate no parent wants for their child.
More Must-Read Stories From TIME