It was thought to be almost eradicated from the surface of the globe, stored in the archives of Western health authorities. Yet polio has not disappeared. On July 21, the United States recorded its first case of poliomyelitis in nearly a decade. A young man hospitalized for paralysis of the leg and tested positive for polio, in Rockland County, a few dozen kilometers from Manhattan. Traces of the virus were later found in sewage from that county and another nearby.
An isolated case ? Probably not, fear the American authorities. In mid-August, the virus was this time detected in New York’s wastewater, “suggesting that it is probably circulating locally”, warned the city’s health authorities. And in recent months, other regions of the globe have been affected. What raise the specter of a resurgence of this extremely contagious disease in France?
Several pockets of resurgence on the globe
In the United States, “the detection of the virus in samples of New York wastewater is worrying, but not surprising,” commented Mary Bassett, a state health official. What worries him is that “for every case of the disease identified, several hundred could remain undetected”. Polio, an extremely contagious disease that affects children under 5 in particular, can cause permanent paralysis of the limbs, and even death in some cases. It is caused by the poliovirus, of which there are several forms.
Under the impetus of national governments, the World Health Organization (WHO) or UNICEF, the launch in 1988 of the Global Polio Eradication Initiative (IMEP), accompanied by a massive deployment of vaccination, has virtually eradicated it worldwide. “The number of cases due to wild poliovirus has decreased by 99% since 1988, from an estimated 350,000 cases in more than 125 endemic countries to only six cases reported in 2021”, reports the WHO. The only exceptions are Afghanistan and Pakistan, where an endemic strain of wild poliovirus is spreading.
However, as we have said, since the beginning of the year, several pockets of resurgence have been uncovered. In June, British authorities announced that they had detected traces of a form of polio in eight London boroughs, with “genetic diversity” suggesting “transmission of the virus”, although “no one has been diagnosed with the virus “, reassured the Minister of Health, Steve Barclay. In Israel, nine children have tested positive since March, eight of whom are asymptomatic. However, people who do not develop symptoms can still transmit the disease. And in February, a case was detected in Malawi. “The first case of wild poliovirus in Africa for more than five years” indicated the WHO, specifying that it was “a case imported from Pakistan”. Problem, “as long as the wild polio virus exists somewhere in the world, all countries are exposed to the risk of importing it”, according to Dr. Matshidiso Moeti, WHO Regional Director for Africa.
The oral vaccine in the viewfinder
The only solution to eradicate this disease for which no treatment exists: vaccination. To date, “two vaccines are available: the oral poliomyelitis vaccine (OPV) and the inactivated poliomyelitis vaccine (IPV), adds the WHO. Both are effective and safe”. But one raises another problem: OPV can cause contamination in unvaccinated people.
How ? “A vaccine-derived poliovirus is an attenuated strain of poliovirus originally included in OPV that has evolved over time by genetic mutation to become virulent and behave more like a wild virus,” explains Public health France. As a result, it can be transmitted to people who are not properly vaccinated against poliomyelitis and cause the disease mainly in countries using the oral vaccine. Just like the viruses contained in the oral vaccine, they are excreted in the stools of vaccinated individuals”. And end up in the sewage.
The case of the patient identified in New York State thus indicates “a chain of transmission from an individual who received OPV”, according to New York health officials. A vaccine that has not been used in the United States since 2000, which suggests that the origin of the virus could come “from a place outside the United States where OPV is administered”. Moreover in London, the traces of polio found in sewage samples do not correspond “either to a strain of virus in the wild state, but to a” derived “version which is used to vaccinate against the disease” , according to the British authorities.
Gaps in vaccination coverage
“The risk for New Yorkers is real, but defending yourself is simple: get vaccinated,” said Ashwin Vasan, city health officer. There, as in London, the vaccination of children has been reinforced and the authorities have urged residents who are not vaccinated to do so. And there is urgency: only 86% of young New Yorkers aged 6 months to 5 years have received three doses of the vaccine, according to the authorities.
And the Big Apple is far from the only one concerned. The Covid-19 crisis and the lack of access to care and information have caused the largest continuous decline in childhood vaccination for nearly thirty years, the UN worried when it published in mid-July. a joint report by WHO and UNICEF. According to the document, the proportion of children who received all three doses of vaccine against diphtheria, tetanus and poliomyelitis (DTP)the benchmark for global vaccination coverage, fell from 86% in 2019 to 81% in 2021.
Some 25 million children missed one or more doses of DTP in 2021, i.e. 2 million more than in 2020, and 6 million more than in 2019. Almost three quarters did not even receive any dose, the majority being in countries of the South with medium or low incomes such as India, Nigeria, Indonesia, Ethiopia or the Philippines. Hence the importance of setting up vaccination campaigns “to catch up with the children missed out”, urges Dr. Kate O’Brien, WHO Director for Immunization, Vaccines and Biological Products.
France well vaccinated, but…
And in France, are we sufficiently protected? “The country enjoys a vaccination coverage very high vis-à-vis poliomyelitis: 99% for the primary vaccination and 96% for the booster in infants in 2019”, reassures Public Health France. A vaccination that relies on the use of IPV, which does not present the risk of mutation and transmission of the virus associated with OPV.
However, “vaccination coverage has its pockets of heterogeneity,” underlines Antoine Flahault, epidemiologist and director of the Institute of Global Health at the Faculty of Medicine from the University of Geneva. Thus, in 2004, it was only about 75% at the age of 2 years, and not 96%, in the departments of Creuse, Lot-et-Garonne and Guyana. This puts these populations at risk in the event of re-emerging viral circulation”. Many French people are not up to date with their reminders.
Faced with the potential risk, “the health authorities remain vigilant with regard to the risk of introduction of strains of poliovirus derived from the vaccine linked to people from countries in which OPV is still used, assures Public Health France. Clinical and biological monitoring coordinated by the National reference center of Enteroviruses and Parechoviruses makes it possible to detect any clinical suspicion”. For Antoine Flahault, there is “urgency, even in France, to search for the virus in wastewater, in order to possibly be able to relaunch the vaccination of insufficiently covered children if the virus were to be detected there”. According to Public Health France, “the last case of autochthonous poliomyelitis dates back to 1989”.