Vaccines helped neutralize the global threat of polio, a dreaded scourge that can cause paralysis and death, but that life-saving drug may have led unintentionally to the virus’ reappearance this year in the U.S. and other countries.
A form of the vaccine taken by mouth that uses a weakened live version of the poliovirus is believed to have spawned the virus that health officials say is spreading in two suburban New York counties and infected and partly paralyzed an unvaccinated young adult there.
“Even a single case of paralytic polio represents a public health emergency in the United States,” the Centers for Disease Control and Prevention said in a new report on the case Tuesday, noting that it is only the second U.S. case of community transmission of the virus since 1979. “This case highlights the risk for paralytic disease among unvaccinated persons.”
Fortunately, those who have been vaccinated against polio — it’s among 10 immunizations required for admission to California K-12 schools — should not be concerned, medical experts say. The live version of the virus used in vaccines now in question is no longer used in the United States.
“If you’ve been vaccinated, you’ll be protected. You have nothing to worry about,” said Dr. Lee Riley, who chairs infectious diseases and vaccinology department at the University of California, Berkeley. “It’s those people who’ve not been vaccinated who need to be concerned.”
The polio virus enters the body through the mouth and lives in the throat and intestines. It is spread through person-to-person contact by coughing or sneezing or through contact with infected feces. Most people who become infected experience no symptoms. About one in four develop influenza-like symptoms that go away after two to five days.
Two similar cases have been identified in the United Kingdom and Israel. As of Aug. 10, the CDC report said, no other cases of polio have been detected in the U.S.
But the CDC said detection of polio virus in the wastewater of New York’s Rockland and Orange counties “indicates community transmission and ongoing risk for paralysis to unvaccinated persons.”
How the New York patient got infected and the virus got into the wastewater system isn’t known, the CDC said. But because he hadn’t traveled overseas, it “indicated a chain of transmission within the United States originating with a person who received a … oral polio vaccine abroad.”
The U.S. stopped administering oral live attenuated polio vaccines for routine vaccination in 2000 and they were largely discontinued globally in 2016, the CDC said. Since then the U.S. has used only polio vaccine containing inactivated virus that cannot replicate, strengthen or cause disease.
But vaccine-derived polio viruses can still emerge when live, weakened or “attenuated” versions of the virus are administered in oral polio vaccines in places with low vaccination coverage, the CDC said.
When the virus reproduces in a person who was recently vaccinated, “it can result in viral reversion to neurovirulence,” the CDC said, meaning it regains its potency as a paralyzing and potentially lethal pathogen. Unvaccinated people exposed to the vaccine-derived virus can then become sick with polio.
“It just goes back to what it was,” Riley said. “That type of occurrence is rare.”
About 1 in 2,000 infected people will experience weakness in the arms and legs. As many as 1 in 10 people may suffer more extensive paralysis that makes them unable to breathe on their own, and many of them die.
The U.S. saw its worst polio outbreaks in the 1940s before vaccines were available, when as many as 35,000 cases of polio paralysis occurred each year. The outbreaks prompted quarantines and made many parents fearful of letting their kids play with other children outside.
Cases fell dramatically with the introduction of polio vaccines in the 1950s, and North America and South America were declared polio-free in 1994. The last similar transmission of the virus in the U.S. was in 2005 when an unvaccinated Amish infant with a weakened immune system was hospitalized in Minnesota.
No paralytic disease resulted, and there were no other cases. It was considered the first demonstrated transmission of vaccine-derived polio virus in an under-vaccinated community in a developed country.
The young unvaccinated adult infected last month in New York initially experienced fever, neck stiffness, gastrointestinal symptoms and limb weakness that prompted a visit to the hospital.
What was especially concerning to the CDC was the relatively low vaccination rate in those New York counties. In Rockland County, where the infected patient lives, the polio vaccination rate had fallen from 67% in 2020 to 60% in 2022 and as low as 37% in some ZIP codes, compared to a national rate of nearly 93%.
“Low vaccination coverage in the patient’s county of residence indicates that the community is at risk for additional cases of paralytic polio,” the CDC report said.
Do people vaccinated years ago need to worry, given that some older people vaccinated as children against smallpox have recently contracted the related monkeypox virus? COVID-19 vaccine protection has faded so rapidly that health officials have ordered repeated boosters.
“They all wane over time,” Riley said of vaccines, “but not as rapidly as the current COVID vaccines.”
Dr. Seth Blumberg, an assistant professor of medicine at UC San Francisco, said there are likely more undetected infections among the unvaccinated in New York because most people with polio don’t suffer symptoms. The cases are on the other side of the country, but it’s reason enough to get the shots.
“If you’re unvaccinated in California, I would get vaccinated,” Blumberg said. “If you vaccinated, you are quite safe.”