Polio is back in rich countries, but it poses a much bigger threat to the developing world

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That’s how this year’s closely related polio outbreaks in New York state, London and the greater Jerusalem area could have started. A child in Afghanistan or Pakistan received two drops of Albert Sabin’s oral polio vaccine (OPV), which contains a weakened live virus, in December 2021 or so. Shortly thereafter, while the boy was still shedding some of the virus in his stool, his family traveled to the UK, where the vaccine virus found fertile ground in an unvaccinated Orthodox Jewish community in London and began to circulate from person to person. person. Somewhere along the way, it also began to change, detecting mutations that can turn the vaccine virus into one that, on rare occasions, can cripple.

That virus then jumped to Israel and an Orthodox Jewish community in Rockland County, northwest of New York City, says Nicholas Grassly, an epidemiologist at Imperial College London and a member of the UK’s National Poliovirus Containment Authority. . He reconstructed the “plausible” scenario based on the epidemiological timeline and the viral sequences detected in the wastewater. In Rockland County, an unvaccinated young man from the Orthodox community sought medical attention for weakness in his legs in June, the first case of polio in the US in a decade.

The ongoing outbreak underscores the risks facing unvaccinated and undervaccinated people, even in rich economies. All three countries have increased vaccinations, and on September 9, New York Governor Kathy Hochul declared a state of emergency in an attempt to curb the outbreak.

But Grassly and other polio experts stress that large outbreaks of paralytic polio cases remain highly unlikely in rich countries, thanks to high vaccination coverage and good sanitation. “There is a risk that we will end up reporting one or two cases in London,” says Grassly. Mark Pallansch, a polio virologist who recently retired from the US Centers for Disease Control and Prevention (CDC), believes the same is true for New York state. Both are far more concerned about similar outbreaks in low-income countries, which receive far less press coverage but have already paralyzed nearly 300 children this year, mostly in Yemen and Africa, and about a resurgence of wild poliovirus in Africa.

OPV remains the workhorse of the global eradication program because it is cheap, easy to use, and confers robust gut immunity that helps stop polio transmission. But where immunization rates are low, the vaccine virus can continue to spread from person to person and, over time, acquire enough mutations to regain its ability to paralyze, just like the wild virus. As few as six nucleotide changes in the region that codes for a viral capsid protein called VP1 are enough to transform a harmless Sabin virus into what is known as a vaccine-derived poliovirus (VDPV). That’s why rich countries use Jonas Salk’s inactivated polio vaccine (IPV) instead, which must be injected. That vaccine cannot be reversed.

Most VDPV outbreaks are caused by one of the three polioviruses, type 2. Because type 2 has been eradicated in the wild, that component of the vaccine has been removed from general use and is deployed only to combat type 2 outbreaks. Both Afghanistan and Pakistan used type 2 OPVs during mass campaigns in December 2021.

The viruses that arrived in New York, London, and Israel did not arrive as complete VDPVs; they were only halfway along their journey, with only a few genetic changes from Sabin type 2 virus. In London, Grassly says, sequence analysis of virus samples collected from sewage shows a gradual evolution to a VDPV between February and June. . Through retrospective analysis, New York state health authorities found traces of a type 2 Sabin-like virus in wastewater collected to look for SARS-CoV-2 since April. The virus that paralyzed the young man in June had 10 nucleotide changes in the critical VP1 region. New York officials continue to find Sabin-like viruses in a growing number of counties, some with a few nucleotide changes, others complete VDPVs.

Vaccination rates are high in the US and UK overall, but low in some communities. In Rockland County, where anti-vaccination sentiment is high, only 60% of children under the age of 2 had received the full three doses of IPV by August. In a zip code, coverage is only 37%. (Rockland was also the site of a major measles outbreak in 2019 that nearly cost the United States its measles-free status.) The June polio case is “tragic but totally predictable and preventable,” says Pallansch.

The state has launched an all-out effort to vaccinate children with IPV. Although excellent at preventing paralysis, IPV isn’t as good as OPV at stopping flare-ups, but experts in the UK and US think it can probably get the job done. The idea is to build a wall of immunity around the virus and then test how well it works, says Andrew Pollard, director of the Oxford Vaccine Group and chairman of the UK’s Joint Vaccination and Immunization Committee. “There are reasons to think that it will [work]. So end of story.

So far, New York state has had limited success in vaccinating the most resilient communities, says Rockland County Health Commissioner Patricia Schnabel Ruppert. But even in areas where vaccine coverage remains low, the virus may die out because the susceptible population won’t be large enough to sustain the spread, Grassly says.

Also, “The kind of sanitation problems we see [in poor countries] they just don’t exist” in the wealthy, says Aidan O’Leary, head of the Global Polio Eradication Initiative. Poliovirus spreads easily through fecal-oral contact and thrives in places where sanitation is lacking and clean water is scarce. In high-income countries, the main route of transmission is respiratory, which is less efficient, says Pallansch.

If IPV vaccination is not enough, contingency plans are being made both in the US significantly less likely to return to its neurovirulent form. The vaccine is now being used in 22 low-income countries battling polio outbreaks under an emergency use authorization from the World Health Organization. But the US and UK would have to jump through huge regulatory hurdles to get approval to use nOPV2. “We are a long way from reaching” nOPV2, says Janell Routh, who is leading the CDC’s investigation of the New York case.

Vaccine-derived polioviruses are not the only threat to the global eradication effort. Cases caused by wild poliovirus have also increased: Pakistan has already reported 17 this year, up from one in all of 2021. After being confined for years in Afghanistan and Pakistan, the wild virus jumped to Africa in 2021, where it was last seen. 5 years ago. It has already paralyzed six children in Malawi and Mozambique. The risk of further spread is high, says O’Leary. Mozambique is also battling outbreaks of type 2 and type 3 vaccine-derived strains.

The global priority remains “stopping polio at its source,” while New York state and London deal with “spillover” effects, says O’Leary. With increased surveillance and update vaccination campaigns, “They’re doing everything right at the right time,” he says.

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