The silent return of polio

The race to worldwide polio eradication risks running aground meters from the finish line. Only at the beginning of the year did cautious optimism prevail over the possible global cancellation of this highly contagious viral disease, once responsible for millions of deaths a year.

Thanks to vaccines, global polio cases have dropped by 99%, from 350,000 in the late 1980s to 240 cases this year. The wild poliovirus remains endemic only in Pakistan and Afghanistan, which however reported, at the beginning of 2022, just four cases in total. However, in recent months, signs of a possible return of polio have surfaced in several countries, including some long thought to be free of the infection.

Unwelcome return. In February 2022, Malawi announced its first case of polio in 30 years, after a 3-year-old girl became infected with the virus probably imported from Pakistan: it was the first wild poliovirus infection in Africa in 5 years. part. Pakistan in turn reported 14 cases, 8 of them in the same month last spring. In March, polio reappeared in Israel, where it hadn’t been seen since 1988.

The virus in the sewers. In June, epidemiological surveillance bodies in the UK discovered traces of the polio virus in several water samples taken from the London sewers. The last case of polio in the UK dates back to 1984 and at the moment no new infections have been reported, but even if the general risk remains low, the authorities have invited the population to verify that they have carried out the entire course of four polio vaccines to reduce the chances of transmission.

In the first half of August, the same alarm came from the New York sewers; three weeks earlier, a poliovirus infection had paralyzed a man living in Rockland County, north of the Big Apple.

Where does the pathogen come from? The virus isolated in London is of the same type as the attenuated poliovirus used in oral polio vaccines, administered in mass vaccination campaigns in developing countries.

As explained by the Istituto Superiore di Sanità, there are in fact two types of polio vaccines, “the” inactivated “Salk (IPV), to be administered by intramuscular injection, and the” live attenuated “Sabin (OPV), to be administered via oral. Sabin’s vaccine, administered up to recent years also in Italy, has made it possible to eradicate polio in Europe and is recommended by the World Health Organization in its campaign to eradicate the disease worldwide ”. But people immunized with it can briefly disperse the attenuated virus through their feces.

This poses a potential risk in countries where the vaccination campaign is slow and not homogeneous, or in neighborhoods of theoretically “zero risk” countries where there are pockets of the population not vaccinated against polio. In Great Britain and the United States, as well as in Italy, an injectable vaccine based on an inactivated virus that does not spread through the faeces is used – this is why the viruses found in the sewers are thought to come from an individual vaccinated with the vaccine. oral, or from an unvaccinated who became infected with a vaccine-derived virus while abroad.

skepticism and pandemic. All these patchy cases suggest that the virus, in its wild or vaccine-derived form, has been circulating under the radar for at least a year, thanks to the covid pandemic that has blocked mass vaccination campaigns for months. There are also other factors to slow down the efforts: political (in Afghanistan the Taliban have long opposed the polio vaccine), social (the nomadism of some populations), geographical (the inaccessibility of some territories) but also ideological: in Pakistan and Afghanistan there is a widespread belief that vaccines are a western weapon to sterilize the population.

Mission possible. Polio is also difficult to eradicate because it spreads silently, causing flu-like symptoms. But in one out of 200 infected it leads to irreversible paralysis: when a case of paralyzing polio emerges, it therefore means that there are also hundreds of other untracked cases.

In order to be able to eradicate polio by 2026, as foreseen by a plan of the Global Polio Eradication Initiative, the transmission of both wild poliovirus strains and the vaccine-derived virus must be stopped. But it is also necessary to improve living conditions in countries where the virus is still present: poliovirus, even in its attenuated form, spreads more easily where access to clean water and adequate sanitation are not guaranteed.