The discovery of a case of polio in the United States and traces of the virus in sewage in the United Kingdom and Israel raise fears of a resurgence of this disease declared as eradicated on a large part of the planet. A reminder of the importance of vaccination in the fight against poliomyelitis, for which there is no cure.
“At some point, polio eradication seemed within reach. Then the hard law of public health was reminded of us: we must never let up on our efforts”, affirms Maël Bessaud. This poliovirus specialist at the Pasteur Institute does not stop hammering the same instruction: “Let’s vaccinate and let’s stay vigilant. In Western countries, polio seems distant, non-existent, but it is still there.”
End of July, a case of poliomyelitis in the United States, the first in nearly a decade, came as a wake-up call. The affected man, aged 20 and residing in Rockland County, a suburb of New York, had gone to the hospital because of paralysis in the leg. The diagnosis came quickly. He was not vaccinated. Today, he still suffers from partial paralysis.
This highly contagious disease, caused by a virus that invades the nervous system and can cause permanent paralysis or even death, is nevertheless considered eradicated on most of the planet. In 1988, there were about 350,000 cases per year, mostly in children under five, in 125 countries. Today, that number has dropped to 99%. “Incredible progress that we owe to the massive vaccination campaigns launched by the Global Polio Eradication Initiative”, greets Maël Bessaud. The virus continues to circulate endemic in only two countries: Afghanistan and Pakistan.
A vicious vaccine cycle
So how to explain that this young American caught a disease almost forgotten in Western countries? “Because as long as the virus is not 100% eradicated, it will continue to circulate”, simply comments the specialist.
First possible scenario: the patient went to a country where the virus is still circulating, or he was in contact with a patient who was returning from there. “Especially since the poliovirus, which is transmitted faecal-oral through water, soiled food or a child’s diaper, for example, has a specificity: it only causes paralysis in one patient out of About 200”, explains Maël Bessaud. “This means that the number of people likely to carry it is much greater than the number of actual patients.” That’s what happened recently in Malawi and in Mozambique. Two children, not up to date with their vaccination, were infected with a strain of the virus from Pakistan, transmitted hidden through an asymptomatic person.
In the case of this American patient, genetic analyzes have however made it possible to rule out this track. This time, the explanation lies in a pernicious effect of vaccination. Two types of vaccines have been deployed around the world: one, by injection, is the most widespread in developed countries; the other, oral, is used mainly everywhere else. “The first uses an inactivated vaccine”, explains Maël Bessaud. “It protects against the disease but not against the virus. You can therefore be a carrier without knowing it and be a danger if you come across an unvaccinated person.”
“The oral vaccine has the advantage of being easy to administer, but above all, it protects against the disease and prevents being contaminated by the virus. It is therefore better for avoiding human-to-human transmission”, he continues. . “But in his case, we are using a ‘attenuated’ poliovirus, which is harmless but still alive.” In the weeks following his vaccination, the patient will therefore excrete this virus in his stools and therefore in the environment.
Nothing serious if everyone around him is also orally vaccinated. “But if he’s in a community that’s not vaccinated, or if he’s traveling to an area where only injection vaccine is used, the virus may start circulating again.” And this is where the problem can arise: if it spreads for several months, it can, through mutations, become virulent again. “This is what happened for this American patient. He was exposed to this strain of the virus derived from an oral vaccine, and he developed symptoms because he was not vaccinated”, summarizes Maël Bessaud.
Worldwide, 698 cases of poliomyelitis linked to vaccine-strain derivatives were detected worldwide in 2021, according to the WHO, in unvaccinated people alone.
A virus that circulates unknown
“This patient therefore comes to remind us that even if we feel protected, the virus is still there and the only way to protect ourselves from it is to get vaccinated”, insists the poliovirus expert again. Proof of this is, in mid-August, US Centers for Disease Control and Prevention (CDC) announced that the strain of poliovirus which infected the young man had been detected in several samples of wastewater taken between May and July in the county of Rockland where he is from, in that neighboring Orange and in New York.
And the United States is not the only ones concerned. In the United Kingdom, the alert was given as early as June. Traces of poliovirus have been found in sewage from eight London boroughs. “And the analyzes show that these strains collected are linked to those found in the United States, and also to others taken in Jerusalem”, notes Maël Bessaud. “Not only is the virus circulating well, but it is crossing borders.” In response, the United Kingdom, where vaccination against polio is not compulsory unlike France, offered an injection to all London children aged 1 to 9 years.
“For the majority of the population, vaccinated, there is little risk. But this increases as soon as we enter neighborhoods or within communities where the vaccination rate is low”, notes the specialist. “It also shows the importance of keeping up to date with booster shots.” In France, the first injection takes place at two months, then four and eleven months. It is then necessary to carry out reminders at 25, 45, 65 years and then every ten years.
The concern is all the more acute, for Maël Bessaud, that the Covid-19 crisis has led to the largest decline in childhood vaccination for nearly thirty years, according to the UN. According to a report published in July, the proportion of children who received all three doses of the vaccine against diphtheria, tetanus and poliomyelitis (DTP) fell from 86% in 2019 to 81% in 2021. In total, some 25 million children missed one or more doses of polio vaccine in 2021.
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>> To read also: “Vaccination against measles in France, collateral victim of Covid-19”
Hope for a new vaccine
Will we ever succeed in completely eradicating polio from the entire planet? “Unfortunately, I fear that we are reaching a ceiling in vaccination,” laments the specialist from the Institut Pasteur. “Certain areas in the world are too difficult to reach for security reasons, especially in Africa. In others, notably in Pakistan and Afghanistan, we are faced with a population that categorically refuses to vaccinate their children,” he explains. “And without complete vaccination, the disease will never be eradicated.”
Despite everything, the specialist wants to be optimistic. “The virus in its wild form is losing ground. In 2021, only six cases have been identified,” he says. And a new oral vaccine, which should limit the risk of the virus becoming pathogenic again, is currently being tested. “Currently, it is administered in twenty countries. In one or two years, we will see if it works,” he concludes. “And then we can hope to approach zero cases.”