Polio is returned in wealthier countries, but it threatens developing nations more.

                          

Pop-up polio clinic in Rockland County, New York.

New York has declared a state of emergency, but experts think a nationwide pandemic is highly improbable.

These are the possible origins of the polio epidemics that hit New York state, London, and greater Jerusalem this year. In about December of 2021, a youngster in Afghanistan or Pakistan was given two doses of oral polio vaccine (OPV) developed by Albert Sabin, including a weakened, live virus. The child's family then moved to the United Kingdom, where the vaccination virus flourished in London's unvaccinated Orthodox Jewish community. The infection spreads swiftly. It also began evolving, acquiring mutations that can turn the vaccine virus into a paralysis-causing virus.

Epidemiologist Nicholas Grassly of Imperial College London and the U.K. National Authority for the Containment of Poliovirus said the virus moved to Israel and Rockland County, northwest New York City. Using the epidemiological timeframe and the viral genomes found in the sewage, he pieced together a "plausible" scenario. In June, an unvaccinated Orthodox young man from Rockland County sought medical attention for leg weakness.

The ongoing outbreak highlights the dangers unvaccinated and under-vaccinated persons face even in prosperous societies. New York Governor Kathy Hochul declared a state of emergency on September 9 to tackle the pandemic.

In contrast, Grassly and other polio experts stress that large outbreaks of paralytic polio infections remain improbable in developed nations due to high vaccination coverage and good sanitation. Grassley warns that "there is a danger we will end up reporting one or two cases in London." Recently retired polio virologist from the United States Centers for Disease Control and Prevention (CDC) Mark Pallansch agrees that the same holds for New York. A reappearance of the wild poliovirus in Africa is of more concern to both parties, as are similar outbreaks in low-income nations, which receive much less press coverage but have already crippled around 300 children this year, primarily in Yemen and Africa.

Because it is low-cost, simple to administer, and provides long-lasting gut immunity, OPV remains a mainstay in the worldwide eradication program's efforts to eradicate polio. However, in areas with low immunisation rates, the vaccine virus can continue to transmit from person to person and regain its paralysing capacity with enough time and mutations, precisely like the wild virus. Vaccine-derived polioviruses are created by making as few as six nucleotide alterations in the area encoding a viral capsid protein termed VP1 (VDPV). Developed nations use Jonas Salk's injectable polio vaccine (IPV). Vaccination is irreversible.

One of the three types of poliovirus, type 2, is responsible for the vast majority of VDPV outbreaks. However, now that type 2 has been eliminated in the wild, the vaccine no longer includes protection against this strain and is now used exclusively in the event of an epidemic. In the December 2021 campaigns, Afghanistan and Pakistan utilised OPV of type 2.

Viruses that made their way to New York, London, and Israel were not yet fully formed VDPVs; instead, they were just halfway there, having undergone only a few genetic alterations from the Sabin type 2 virus. Grassley explains that London's wastewater virus evolved into a VDPV through careful sequencing of the virus's DNA between February and June of this year. New York state health officials have discovered, through retroactive analysis of wastewater collected to look for SARS-CoV-2 as early as April, signs of a Sabin-like virus of type 2. In the vital VP1 area, the virus that paralysed the young guy in June contained ten nucleotide alterations. Authorities in New York are discovering Sabin-like viruses in more and more counties, some with minor nucleotide modifications and others as full-fledged VDPVs.

Vaccination rates are high in the United States and the United Kingdom, yet they are low in specific areas. In August, just 60% of children under the age of 2 in Rockland County had received all three IPV doses despite the county having a high rate of antivaccination sentiment. Only 37% of the population in one zip code is covered. (In 2019, a large measles outbreak in Rockland nearly cost the United States its measles-free status.) Pallansch said the polio case in June was "tragic but entirely predicted and avoidable."

The state vaccinates children against IPV. IPV isn't as successful as OPV at preventing epidemics, but U.K. and U.S. scientists believe it can do the job. l.The idea is to develop a barrier of immunity around the virus and measure its efficacy, says Andrew Pollard, director of the Oxford Vaccine Group and chair of the U.K. Joint Committee on Vaccination and Immunisation. It's hoped-for. Thereby ending the tale. Patricia Schnabel Ruppert, the health commissioner for Rockland County, New York, has had mixed results in immunising the state's most resistant communities. However, Grassly argues that the virus may exhaust itself even in regions with low vaccine coverage because the number of people vulnerable to infection will be too small.

Moreover, Aidan O'Leary, the Global Polio Eradication Initiative chairman, said, "The kind of sanitary concerns that we see [in poor nations] simply don't exist" in wealthier countries. Poliovirus spreads rapidly through faecal-oral contact, especially in places with poor sanitation and insufficient safe drinking water. However, Pallansch notes that the respiratory route is the most common in high-income countries despite its inefficiency.

Suppose IPV vaccination is insufficient to prevent an outbreak. In that case, the U.S. and U.K. are preparing to test a new vaccine, novel OPV2 (nOPV2), which is supposed to be as successful as OPV in preventing outbreaks while being less likely to revert to its neurovirulent form. The vaccine has been sent to 22 low-income nations to battle polio outbreaks since getting an emergency use licence from the WHO. To acquire approval to deploy nOPV2, however, the United States and the United Kingdom would have to jump through significant regulatory hoops. Janell Routh, heading the CDC's study into the New York case, said, "We are far from reaching for" nOPV2.

The global endeavour to eradicate polio has multiple challenges, not the least of which are polioviruses bred in vaccines. Wild poliovirus cases are also on the rise, with 17 reported in Pakistan this year compared to just one in all of 2021. Years after being contained in Afghanistan and Pakistan, the wild virus jumped to Africa in 2021, where it had not been spotted for five years. Six youngsters in Malawi and Mozambique have been paralysed as a result. O'Leary claims there's a significant chance of further dispersal. Mozambique is fighting off epidemics of both types 2 and 3, including vaccine-derived strains.

According to O'Leary, "stopping polio at the source" is still a key priority worldwide, while New York and London deal with the "spillover" repercussions. As a result of increased monitoring and catch-up immunisation drives, "They are doing everything right at the appropriate time," he says.