Reinfections are not a new phenomenon, recalls from the outset Catherine Hankins, professor of public health and population health at McGill University and co-chair of the Task Force on Immunity to COVID-19. We have known since very early in the pandemic that it existed, but it was relatively rare, especially when the viruses that reinfected were similar.
However, the arrival of Omicron has increased the number of reinfections. According to the latest data from the UK, the risk of re-infection is now 10 times higher than during the waves caused by Delta.
Alain Lamarre, professor-researcher specializing in immune responses and virology at the National Institute for Scientific Research (INRS), specifies that the genetic diversity of Omicron makes it easier to escape the immunity conferred by infection or vaccination.
It’s unclear whether the 90-day period still holds in the case of the BA.2 subvariant, which accounts for 75% of COVID-19 cases in the country. Moreover, the latest report (New window) on reinfections of the National Institute of Public Health of Quebec (INSPQ) specifies that it is too early to draw conclusions. It’s still a fairly new variant and we don’t understand everything yet
recalls Mr. Lamarre.
In addition, the meteoric amount of infections in recent months is probably contributing to the increase in the number of reinfections, believes Mr. Lamarre.
” The more cases there are, the greater our chances of infection and re-infection. »
In the UK, the latest data (New window) show that in mid-April, nearly 12% of cases would be reinfections. Of the 900,000 cases of re-infection documented in the UK since the start of the pandemic, just over 10,000 people have been re-infected three times and only 100 have been re-infected four times.
Public health in France (New window) considers that reinfections with SARS-CoV-2 are not uncommon events, particularly in a context of high circulation of the Omicron variant
. More than 650,000 cases of reinfection with SARS-CoV-2 have been identified since December 6, 2021 in France. From March 13 to 22, 5.4% of all cases were reinfections.
France does not count reinfections occurring within less than 60 days, but indicates that they are rare
. Since the beginning of December, 8.4% of reinfections have occurred between 60 and 89 days after the first infection; 5% between 90 and 119 days; 18.6% between 120 and 179 days; 59.9% between 180 and 364 days and 8.1% for 365 days or more.
In Quebec, the latest report from theINSPQ (New window) indicates that there were approximately 8,800 reinfections between November 28, 2021 and November 1er January 2022, i.e. approximately 5% of the cases detected during this period. About 16% of these reinfections occurred less than 150 days before the first infection.
Remember that the number of tests is greatly restricted, which has an influence on the confirmed number of infections and reinfections. It is for this same reason that theINSPQ no longer provides new data on reinfections.
Why are we talking about 90 days?
This 90-day period was established by the World Health Organization (WHO) and by the Centers for Disease Control and Prevention (CDC) in the USA. In general, an infection, after 90 days, it is no longer detected
says Mr. Lamarre.
He explains that, in the first 90 days after an infection, it is difficult to know for sure if the infection is indeed a reinfection.
As shedding of SARS-CoV-2 can persist for several weeks, it can be complicated to know if it is a prolonged infection, a reactivation of the initial episode or a reinfection, indicates moreover theINSPQ.
[L’infection] may vary over time. The virus can pass from the throat and then go to the lungs. The infection can have waves or phases during which one is more or less symptomatic. It’s hard to rely on symptoms alone
he says.
Within three months, the sample must be sequenced to determine whether they are two different viruses. However, the majority of people do not have access to this data. Thus, it is often difficult to know whether one has been reinfected or not.
This is why most countries use the 90-day period to define reinfection and do not count infections that occur less than three months after a first infection.
So does protection after an infection last at least 90 days?
Recently, researchers in Spain discovered the case of a patient who was re-infected within 20 days. For now, this case seems to be an exception, but as UK and French data show, it is possible to be re-infected before 90 days.
So is a person protected for 90 days after an infection? In fact, the answer is complex and it is difficult to say that everyone infected has 90 days of protection.
Jane Heffernan, a York University disease modeling expert who studies the vaccine immune response against COVID-19 through mathematical models, adds that there are multiple factors that influence each individual’s level of immunity. and which could explain why we see more cases of reinfection.
” It depends on age, comorbidities, time since last dose, type of vaccine, etc. »
For example, older people are more at risk of being re-infected, data from the UK shows (New window). In fact, the risk of reinfection decreases by 6% for every 10 years of age.
Mrs. Heffernan adds that initial infections asymptomatic or with few symptoms do not provide lasting immunity
.
According to CDC and the INSPQ (New window)people with a low viral load or asymptomatic during the first infection have a high viral load during the second episode, suggesting that a weaker immune response was developed during the first infection.
This is why Mr. Lamarre and Ms. Hankins and Ms. Heffernan believe that the population must be aware that there is a risk of reinfection, and sometimes within 90 days.
Less than 60% of Canadians eligible for a booster dose received it.
Photo: CBC/Ben Nelms
Does one infection equal one dose?
No, say the three experts. An infection improves a person’s immunity, but it is often short-lived. The vaccine remains the best protection, says Hankins.
People who are not vaccinated and then infected do not develop a sufficient immune response to prevent reinfection, a recent study indicates. (New window) from Denmark, which has not yet been peer reviewed.
According to the UK government, unvaccinated people were twice as likely to be re-infected, compared to people who received two doses.
According to an INSPQ analysis (New window) published on April 21, a first infection, even without vaccination, reduces the risk of infection with the Omicron variant by 44% and reduces the risk of hospitalization by 81%. On the other hand, the protection conferred by a previous infection increases significantly with each additional dose of vaccine.
It should be noted that in unvaccinated people, protection against reinfection decreases rapidly. It goes from 66% after three months, then to less than 50% after 6 months and less than 30% after 12 months.
In comparison, in a person infected and vaccinated with one or two doses, protection against reinfection goes from nearly 80% to more than 60% after two months, then remains stable for up to 12 months.
In a triply vaccinated person, this protection remains at 80% for 2 to 5 months (impossible to say for a longer period, since the third dose has not been available for six months).
It should be noted that this analysis of theINSPQ does not offer data that demonstrates how well infection with BA.1 protects against infection with BA.2.
One thing is certain, people with one infection and three doses have hybrid immunity, says Lamarre, who adds that immunity is more constant with vaccination.
” It is the best form of immunity. Vaccination gives a fairly homogeneous immunity from one person to another, which is not the case in an infection. »
“Three months is still quite a good scale, but you shouldn’t just rely on an infection to think you’re safe. ad vitam aeternam. Do not hesitate to get vaccinated after your infection, says Alain Lamarre.
The Committee on Immunization of Quebec (CIQ) recommends that people who have been infected receive their booster dose three months after infection, but that these people could receive it as soon as their symptoms end […] and when their period of isolation is over
.
Reference-ici.radio-canada.ca