What do we know about the risk of reinfection with Covid-19?

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In theory, after a primary infection, a patient develops protective antibodies that provide immunity. But in the case of Covid-19, researchers are still wondering about the effectiveness of the antibodies over time and the possibility of reinfection.

The question of reinfection has arisen since the start of the Covid-19 pandemic. In fact, it is difficult to know if the fact of having been infected with SARS-COV-2 implies that one is immune: if the antibodies have developed well, they are not necessarily neutralizing (capable of preventing the development of the disease). Thus, the presence of antibodies indicates that the body has come into contact with the virus, without being certain that it offers protection against a new infection. Even in the presence of neutralizing antibodies, it is difficult to say how long the immunity provided will persist.

Rare reinfections, which exist

With the current state of knowledge, scientists agree that the phenomenon of reinfection is still rare, but a study by researchers from Imperial College London indicates that it is not as much as you might think.

Published in February in the Journal of Infection, its results come from an analysis of 33 recurrent cases of Covid-19 in Brazil confirmed by PCR test, after a disease-free period after the first infection. Researchers say that naturally acquired immunity against SARS-CoV-2 does not guarantee complete protection against subsequent infection. Underlying biological factors were also found: 42% of the cases had an A + blood group. Furthermore, the anti-SARS-CoV-2 antibody analysis for 17 of these cases showed a weaker antibody response after the first infection compared to the control group. Therefore, the recurrence of the infection tended to be more severe.

"This is the largest case series to date that provides evidence suggesting that people who produce a weak antibody response to SARS-CoV-2 may be more likely to be re-infected with the virus in the future. Our results could be important when examining herd immunity thresholds and levels of protection against naturally acquired viruses, ”explains Professor Danny Altmann.

Most frequent reinfections in people over 65 years of age

Researchers from the Statens Serum Institut (Copenhagen) are also among those studying this phenomenon. Their study, published in March 2021 in The Lancet, was based on the 10 million PCR tests performed last year in Denmark - four million Danes have had at least one test. The researchers analyzed the data, focusing on people who tested positive more than once. Their results show that a previous infection with Covid-19 protects most people from reinfection.

Of those who tested positive in the first wave, 0.65% tested positive again in the second wave. By comparison, 3.3% of those who tested negative in the first wave tested positive in the second wave. "Those who had not tested positive before were therefore about five times more likely to test positive later. This corresponds to an 80.5% degree of protection against later infection," the researchers explain. There was no difference in protection between the two sexes, nor between the first part and the last part of the total study period. In other words, "there was no indication that protection had started to decline after six months," the scientific team notes.

However, when dividing the population by age groups, a different trend emerged for the elderly because among those over 65, protection was estimated at only 47%. Which indicates that they are more likely to contract Covid-19 again: People over the age of 65 would be at higher risk of reinfection with only 47% protection against repeated infections, compared to 80% of people of legal age. A finding that highlights the importance of measures such as social distancing and vaccine prioritization.

"Our study suggests that most people will be protected against a new infection for at least six months. But not everyone is protected and especially in the elderly, only half seem to be protected after a first infection," explains the Professor Steen Ethelberg, who conducted the study.

"Even if you have already been infected, our results suggest that it is advisable to continue to follow advice on how to protect yourself from infection. Furthermore, the results highlight the need for vaccination for everyone because natural protection, especially in the elderly, cannot be invoked ".

It is necessary to consider that the study took into account the "classic" Covid-19 strain and not the variants.

In a study published on August 12, 2020 in The Lancet Infectious Diseases by a team of American researchers, cases of reinfection could be explained by:

the fact that the patients were exposed to a very high dose of the virus the second time;

the fact that the patients were exposed to a more virulent strain of the virus (coronavirus variants);

the presence of antibodies due to the first infection (such as dengue).

It is not necessarily a reinfection with the coronavirus.

When an apparently "cured" patient has symptoms of the coronavirus, it is not necessarily a matter of reinfection. Several elements can cause confusion:

failure of screening tests;

the possibility that the samples have been stored incorrectly;

or that the viral load is insufficient to be detected.

But the most likely hypothesis is that the patients in question would suffer a prolonged infection with the coronavirus, not a reinfection. In other words, the virus would still be present in your body, in the form of undetectable traces, and could be "reactivated", due, for example, to a related disease. But the precautionary principle is in order.

8-month protection according to new studies

A study published on February 5, 2021, in the journal Science estimates that immune memory persists for up to eight months after coronavirus infection. 188 patients with asymptomatic, moderate and severe symptoms of the disease (80 men and 108 women) were followed for eight months. Verdict? Neutralizing antibodies (which prevent the coronavirus from infecting cells) persisted beyond six months: 90% of the participants remained seropositive in tests conducted between six and eight months.

Women would be immune longer than men

A team of researchers from Strasbourg recently examined the evolution of the immune response in the months after SARS-CoV-2 infection. Their findings, reported by the National Institute for Health and Medical Research (Inserm) in a press release published on April 12, 2021, indicate that women develop a more stable level of antibodies than their male counterparts.

"Researchers generally found that women had more effective immune protection than men," Insem says.

"Immediately after infection, the level of anti-Covid-19 antibodies is on average lower in women. But over time, a decline follows that is generally less pronounced in women than in men, regardless of their age condition. or weight ".

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