Poliomyelitis wreaked havoc until the advent of vaccination in the late 1950s, but since then has not been rampant in developed countries. However, it is once again prowling close to home: the virus responsible for this sneaky disease was detected this summer in sewage in London and New York State, where it caused a case of paralysis at the end of July.
Why is this pathology, which was being eradicated, resurging in our regions? Are we well prepared to face his return? What about the vaccination coverage of Quebecers against polio? So many questions to which we strive to answer.
How do we explain this reappearance of poliomyelitis in countries like ours?
The polio virus detected in New York and London sewage (and in the person who developed paralysis) has been identified as a so-called vaccine-derived strain. The latter probably appeared in a developing country where vaccination coverage is deficient and where the oral attenuated vaccine is administered rather than the injectable inactivated vaccine used here.
The oral attenuated vaccine consists of live virus. Although weakened, this attenuated material nevertheless multiplies in the intestines of children who receive it, who eliminate it in their stools for several weeks after receiving a dose. These viruses can then circulate in the environment and infect people who have not been vaccinated. By dint of replicating, they end up mutating and can regain their ability to cause an infection that can cause paralysis.
“It is possible that, through a traveler, there was an importation of a vaccine strain of polio which had become virulent again, and when an unvaccinated young adult [à New York] came into contact with this strain, he developed symptoms of paralysis, ”says the DD Caroline Quach, pediatrician and microbiologist-infectiologist at CHU Sainte-Justine.
Are Quebecers well protected against poliomyelitis?
In Quebec, the vaccination of children against poliomyelitis is carried out in four stages: babies are first injected with doses of vaccine at the age of 2 months, 4 months and 12 months, then a dose of recall at the age of four to six years, just before arriving at school. These doses are usually part of a formula combining several other vaccines, in particular against diphtheria, tetanus, whooping cough, hepatitis B and Haemophilus influenza type b.
“This vaccine is very effective, it provides lifelong immunity if the children receive all the doses provided for in the vaccination schedule”, underlines the Dr Nicholas Brousseau, member of the Quebec Immunization Committee.
According to the most recent vaccination coverage survey conducted in Quebec, in 2019, 96.1% of children about to enter school had received the four recommended doses of vaccine. However, the portrait is less clear for adults, because “the vaccination register is more or less reliable” for them, says the DD Quach.
In the past, when polio was present in the country, “this disease was very scary, because people could be paralyzed for their whole life and confined in a steel cage to help them breathe. When the vaccine arrived in the late 1950s, people lined up to receive it. So adults are also very well covered, ”underlines the Dr Brousseau, before recalling that the last case of polio in Quebec dates back to 1995.
What vaccine is administered in Quebec?
As in all developed countries, in Quebec, children are immunized against polio using an inactivated trivalent vaccine administered by injection. The vaccine is said to be trivalent because it contains inactivated viruses from the three wild strains of the polio virus: type 1, which still circulates in Pakistan and Afghanistan; type 2, eradicated in 2015; and type 3, which disappeared in 2019.
This vaccine also protects against vaccine-derived type 2 virus that has been detected in sewage in London and New York State. “The vaccine contains the wild type 2 strain. Even if the virus derived from the vaccine strain is not quite the same, there is cross-immunity that also protects against this virus,” says Dr.r Brousseau.
The DD Judith Fafard, director of the Laboratoire de santé publique du Québec, explains that “an inactivated vaccine is made up of pieces of virus, or virus proteins. So there is nothing that can multiply in our body when we receive this vaccine. An inactivated vaccine is safer than the live attenuated oral vaccine [car la personne vaccinée n’excrète pas de virus vivants qui pourraient infecter d’autres personnes]on the other hand, it is a little less immunogenic”.
Composed of live, but weakened viruses, “the oral vaccine prevents infections causing poliomyelitis, while the inactivated vaccine only prevents complications, only serious forms of the disease”, adds the DD Quach.
What do the symptoms of poliomyelitis look like?
About 70% of infections that occur in children are asymptomatic and therefore go undetected. Asymptomatic people nevertheless excrete viruses in their stools.
About 24% of infected people have mild symptoms: fever, fatigue, sore throat and headache. A type of non-fatal meningitis, characterized by stiff neck and back, as well as severe headaches, can occur in 1% to 5% of cases.
Only one infection in 200 (0.5%) leads to paralysis, the most feared symptom. And among people who develop this complication, 2% to 5% will die if it affects their respiratory muscles.
What would experts recommend if the polio virus were to migrate to Quebec?p
Since the disease is present in New York State, it is not impossible that it will appear in Quebec, experts say. “If the virus arrives here, it is possible that there will be infections – young Quebecers have mainly received the inactivated injectable vaccine, which does not necessarily prevent infections, a bit like the vaccine against COVID. But since we have good vaccination coverage in Quebec, it would not be serious, ”indicates the DD Quach.
Only unvaccinated people run the risk of being very sick: the young New York adult who recently developed paralysis was not, she underlines.
Polio is transmitted mainly by the faecal-oral route, like hepatitis A. “So you have to wash your hands well when you go to the toilet,” explains Dr.D Fafard. “And if parents ever have children whose vaccination schedule is late […], it would be important to ensure that their vaccinations and those of their children are up to date. »