Here’s how this year’s closely related polio outbreaks in New York state, London, and greater Jerusalem might 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. Soon after, when the child was still shedding some virus in their stool, their family traveled to the United Kingdom, where the vaccine virus found fertile ground in an undervaccinated Orthodox Jewish community in London and began to circulate person to person. Somewhere along the way, it also began to change, picking up mutations that can turn the vaccine virus into one that, in rare instances, can paralyze.
That virus then jumped to Israel and to an Orthodox Jewish community in Rockland County, northwest of New York City, says Nicholas Grassly, an epidemiologist at Imperial College London and member of the U.K. National Authority for the Containment of Poliovirus. He reconstructed the “plausible” scenario based on the epidemiological timeline and viral sequences detected in sewage. In Rockland County, an unvaccinated young man in the Orthodox community sought care for weakness in his legs in June—the first U.S. polio case in a decade.
The outbreak, which is continuing, underscores the risks facing unvaccinated and undervaccinated people even in wealthy economies. All three countries have ramped up vaccinations, and on 9 September, 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 big outbreaks of paralytic polio cases remain highly unlikely in rich countries, thanks to high vaccination coverage and good sanitation. “There is a risk we will end up reporting one or two cases in London,” Grassly says. Mark Pallansch, a polio virologist who recently retired from the U.S. Centers for Disease Control and Prevention (CDC), thinks the same is true for New York state. Both are far more concerned about similar outbreaks in low-income countries—which get far less press coverage but have already paralyzed almost 300 children this year, mostly in Yemen and Africa—and about a resurgence of the wild poliovirus in Africa.
OPV remains the workhorse of the global eradication program because it’s 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 encoding a viral capsid protein named VP1 are enough to transform a harmless Sabin virus into what is known as a vaccine-derived poliovirus (VDPV). That’s why wealthy countries instead use Jonas Salk’s inactivated polio vaccine (IPV), which must be injected. That vaccine cannot revert.
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 fight type 2 outbreaks. Both Afghanistan and Pakistan used type 2 OPV during mass campaigns in December 2021.
The viruses that arrived in New York, London and Israel did not come as full-fledged VDPVs; they were only part way along their journey, with just a few genetic changes from the Sabin type 2 virus. In London, Grassly says, sequence analysis of virus samples collected from wastewater shows a gradual evolution to a VDPV between February and June. Through retrospective analyses, New York state health authorities have found traces of a type 2 Sabin-like virus in wastewater collected to hunt for SARS-CoV-2 as early as April. The virus that paralyzed the young man in June had 10 nucleotide changes in the critical VP1 region. New York officials keep finding Sabin-like viruses in a growing number of counties, some with a few nucleotide changes, others full-fledged VDPVs.
Vaccination rates are high in the U.S. and the U.K. overall, but low in some communities. In Rockland County, where antivaccination sentiment runs high, just 60% of children under age 2 had received the full three doses of IPV in August. In one zip code, coverage is just 37%. (Rockland was also the site of a huge measles outbreak in 2019 that almost cost the United States its measles-free status.) The June polio case is “tragic but totally predictable and preventable,” Pallansch says.
The state has launched an all-out push to get children vaccinated with IPV. Although excellent at preventing paralysis, IPV is not as good as OPV at stopping outbreaks, but U.K. and U.S. experts think it can probably do the job. The idea is to build a wall of immunity around the virus then evaluate how well it works, says Andrew Pollard, director of the Oxford Vaccine Group and chair of the U.K. Joint Committee on Vaccination and Immunisation. “There is reason to think it will [work]. So end of story.”
New York state has so far had limited success vaccinating the most resistant communities, says Rockland County Health Commissioner Patricia Schnabel Ruppert. But even in pockets where vaccine coverage remains low, the virus may burn itself out because the susceptible population won’t be large enough to sustain spread, Grassly says.
Besides, “The kind of sanitation issues that we see [in poor countries] simply don’t exist” in wealthy ones, says Aidan O’Leary, head of the Global Polio Eradication Initiative. The poliovirus spreads easily through fecal-oral contact and thrives in places where sanitation is lacking and clean water scarce. In high-income countries, the primary route of transmission is respiratory, which is less efficient, Pallansch says.
If vaccination with IPV isn’t enough, contingency planning is underway in both the U.S. and the U.K. to evaluate the possible use of a new vaccine, known as novel OPV2 (nOPV2), designed to be just as good at stopping outbreaks as OPV but significantly less likely to revert to its neurovirulent form. The vaccine is now being used in 22 low-income countries fighting polio outbreaks under a World Health Organization emergency use authorization. But the U.S. and U.K. would have to jump through big regulatory hoops to get approval to use nOPV2. “We are a long way from reaching for” nOPV2, says Janell Routh, who is leading CDC’s investigation into the New York case.
Vaccine-derived polioviruses are not the only threat to the global eradication effort. Cases caused by the wild poliovirus are also up: Pakistan has already reported 17 this year, up from one in all of 2021. After being confined for years to Afghanistan and Pakistan, the wild virus leapt 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, O’Leary says. Mozambique is also battling outbreaks of type 2 and type 3 vaccine-derived strains.
The global priority remains to “stop polio at the source,” while New York state and London deal with the “spillover” effects, O’Leary says. With ramped up surveillance and catch-up vaccination campaigns in place, “They are doing everything right at the right time,” he says.