Genetic analysis of the virus responsible for the first case of polio in the United States in nearly a decade shows it is linked to vaccine-derived viruses recently detected in Jerusalem and London, the Global Polio Eradication Initiative announced on Friday.
“Further investigations — both genetic and epidemiological — are ongoing to determine possible spread of the virus and potential risk associated with these various isolates detected from different locations around the world,” the program said in a statement.
The analysis indicated the viruses have been circulating for some time, signaling silent spread of vaccine-derived polioviruses over a wide geographic area.
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“It basically suggests there’s been substantial transmission, undetected,” Walter Orenstein, a polio expert at Emory University who was not involved in the analysis, told STAT.
The case, an unvaccinated man in his 20s who lives in Rockland County, N.Y., recently developed paralysis that was diagnosed as having been caused by a type 2 polio vaccine virus. Health authorities in Rockland County, which is north of New York City, said the man had not traveled outside the country in the time when he would have been infected. That means someone else had to have brought the viruses into the country.
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Vaccine-derived polioviruses — VDPVs in polio parlance — come from oral polio vaccine, which contains live but weakened polioviruses. Children who are immunized with this vaccine excrete vaccine viruses in their stools. In places where hygiene is poor, these viruses can spread from child to child, immunizing others as they do. But as they spread, the vaccine viruses can regain the power to paralyze. At this point, more children globally are paralyzed every year by circulating vaccine-derived polioviruses than the wild-type viruses, which now are only endemic in Afghanistan and Pakistan.
Because of the risk associated with the oral vaccine, the United States stopped using it in 2000; it has since exclusively used inactivated polio vaccine, which does not contain live viruses. The United Kingdom uses IPV, as it’s called, as well.
Israel uses both OPV and IPV. But the version of the oral vaccine it uses does not include type 2 viruses, which means this chain of transmission began in yet another, currently unidentified country.
The type 2 component of the oral vaccine was withdrawn in 2016 in a coordinated global effort to try to stop transmission of type 2 vaccine viruses. But the move, known as “the switch,” did not succeed. Until recently, when type 2 vaccine virus outbreaks occurred the response was to use oral vaccine containing only the type 2 strain to try to stop the spread — a fighting fire with fire approach.
It must be from one of these campaigns that the type 2 viruses in this transmission chain originated.
Steve Oberste, chief of the polio and picornavirus branch at the Centers for Disease Control and Prevention, said it is impossible with the information currently available to pinpoint where the transmission chain started or how the viruses that infected the Rockland County man reached him.
The viruses from Jerusalem and London were detected in wastewater sampling. To date there haven’t been paralytic polio cases associated with this chain of transmission reported in either location.
Oberste noted that all three locations — Jerusalem, London, and New York — draw large numbers of travelers.
“Clearly they’re linked. It’s hard to infer directionality, especially when you only have a small number of sequences,” he said. “It’s going to be hard to say: Was it someone who came from Israel directly to New York? Someone who came from London directly to New York? Or even someone who brought it into Israel from somewhere else and it was brought into New York by someone who’s from wherever that place is, but it was the same virus?”
While the finding doesn’t help to solve the mystery of how the Rockland County man contracted polio, it does underscore that countries that have thought polio was a long-gone threat need to ensure vaccination rates stay high and they stay vigilant, the polio program said.
“It is vital that all countries, in particular those with a high volume of travel and contact with polio-affected countries and areas, strengthen surveillance in order to rapidly detect any new virus importation and to facilitate a rapid response,” the statement said.
The Global Polio Eradication Program is a partnership of the World Health Organization, the CDC, the United Nations Children’s Fund (UNICEF), the service club Rotary International, and the Bill and Melinda Gates Foundation.