Covid-19: what do we know about the risk of reinfection?

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In theory, after a primary infection, a patient develops protective antibodies that ensure immunity. But in the case of Covid-19 and with the appearance of new variants, researchers are still wondering about the effectiveness of 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 by SARS-COV-2 implies that one is immunized. If the antibodies have developed well, they are not necessarily neutralizing (capable of preventing the development of the disease). Thus, the presence of antibodies testifies to the fact that the body has been in contact with the virus, without being sure that it offers protection against a new infection. And even in the presence of neutralizing antibodies, it is difficult to say how long the provided immunity will persist.

Majority subvariant BA.2 in Europe: the reason for the high reinfection rate?

Since the beginning of March, Europe has been facing increased contamination with the BA.2 variant, a sub-variant of Omicron. The BA.2 sublineage has become the majority. This increase in the BA.2 variant could be the cause of the many reinfections with Covid-19. It has been indicated that the majority of people who test positive for the BA.2 sublineage today have already been contaminated by the Omicron variant. The ratio of reported possible reinfection cases to all Covid-19 cases has increased again since the end of January.

Do the new variants of Covid-19 affect immunity?

One of the key questions to better predict the evolution of the COVID-19 pandemic is the duration of natural immunity. In theory, a person already infected with a virus is immune to it. When our body comes into contact with a virus, it develops specific antibodies capable of recognizing and fighting the virus more effectively when it comes up against it again.

On the other hand, the appearance of new variants affects immunity due to their differences with the strain with which the patient was previously infected. The BA.2 subvariant in particular shows significant differences from the Omicron variant. These many differences greatly affect the immunity of patients against Covid-19. On the surface of a virus there are several markers: when a person is infected, our immunological memory will rely on these same markers to make antibodies. When a new variant appears, the markers are different, so you have an immune response that is not as effective as fighting the virus and defending the cell that is infected. Explains the specialist Francois Blanchecotte of the Syndicate of Biologists of France to Du Midi.

What is the minimum time between two contaminations with Covid-19?

Immunity to Covid-19 infection is undermined by the emergence of new variants. According to the specialist, the appearance of the BA.2 subvariant has caused an increase in cases of reinfection. The scientist explains that these numerous cases of reinfection are observed in particularly short periods, "sometimes a month to a month and a half" between two contaminations, he points out during an interview for Franceinfo.

Who is most affected by the risk of reinfection by Covid-19?

Certain age groups are more at risk of re-infection with the virus. The population most affected by Covid-19 reinfections are "children first", says the specialist. The scientist explains this phenomenon by the end of the mandatory use of masks in schools. The elderly would also be a population highly affected by this phenomenon of reinfection.

"Many times it is with children, family and in residences for the elderly. The clusters that we discovered are in 57% in residences for the elderly and medical establishments with people with disabilities in 14%. In companies we are in 8% and in nurseries by 10% explained the biologist.

Since the start of the pandemic, several scientific teams have investigated the risk of reinfection after contracting Covid-19 for the first time, and the elderly were already identified as a population at risk. This is confirmed by researchers from the Statens Serum Institut (Copenhagen) who conducted a study on the risk of reinfection taking into account the "classic" strain of Covid-19 and not the variants. Their study published in March 2021 in The Lancet was based on the 10 million PCR tests carried out last year in Denmark: four million Danes had at least one test. The researchers analyzed the data, focusing on people who tested positive more than once. Of those who tested positive in the first wave, 0.65% tested positive again in the second wave.

Dividing the population by age groups, a different pattern emerged for older adults, since among those over 65 years of age, protection was estimated at only 47%. This indicates that they are more likely to be infected again with covid-19: people over 65 would have a higher risk of reinfection with only 47% protection against repeat infection, compared to 80% of people over 65 years old.

An earlier study published in mid-December by Imperial College London indicated that there would be a 5.4% higher risk of being reinfected with Covid-19 with the Omicron variant than with the Delta variant.

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