Covid-19. Why is the number of reinfections exploding in France? We help you see more clearly

“I had Covid-19 on March 5, 2022, can I catch it a second time immediately? » This question is posed to us by Brigitte. West France answers you.

Since December 2021 and the distribution of the Omicron variant in France, the frequency of possible reinfections with Covid-19 has increased markedly. If the risk of reinfection was already present before the Omicron wave in a rather rare way, the first cases of reinfection having been reported from the end of summer 2020, “we know that for some people the protection conferred by the disease is not perfect, not complete and therefore one can be sick a second time with more modest symptoms than the first”, explains Matthieu Revest, infectious disease specialist at the Rennes University Hospital (Ille-et-Vilaine).

After a short stabilization at the beginning of January 2022, the proportion of possible cases of reinfection reported to all Covid cases has been on the rise again since the end of January and now represents 5.4% of all confirmed cases, according to the latest available data published on Friday 1East April by Public Health France.

Possible cases of reinfection are defined as all persons who presented at least two positive tests recorded in the database carried out 60 days apart or more. The time between the two episodes of infection was 242 days on average, said the health agency during its last weekly press briefing.

A reinfection after one month, is it possible?

Contacted, Yannick Simonin, virologist and research professor at the University of Montpellier (Hérault), replies that “Studies show in particular that in people infected with the Omicron variant but not vaccinated, immunity appears weak against the different variants and that it declines quite rapidly over time”.

The Omicron variant tends to induce less severe forms of the disease but more reinfection. In practice, “some people who have recently contracted Omicron (mainly the BA.1 sub-variant) can be reinfected one to two months later, in particular by the BA.2 sub-variant, now the majority in France, continues the specialist in emerging viruses. Although reinfection after one month is not the most likely, it is therefore not impossible depending in particular on your vaccination status and whether you were initially infected with the BA.1 or BA.2 sub-variant. »

How to explain this phenomenon of reinfection?

In all, 685,858 possible cases of reinfection were identified between March 2, 2021 and March 20, 2022, including 95.2% since December 6, 2021, which marks the start of the spread of the Omicron variant in France, reports the health agency in its latest weekly statement. “It seems likely that the attenuation of the post-infectious or post-vaccination immune response within the French population plays a role in this marked increase in the frequency of possible cases of reinfection, in particular in people who have not had booster dose of the vaccine”, indicates Public Health France. And to add that“It is also very likely that the very strong distribution in France of the Omicron variant, characterized by increased transmissibility and significant immune escape, amplifies this phenomenon”.

88% of possible reinfection cases for which a screening result was available and interpretable for the reinfection episode had a result suggestive of Omicron, according to the latest available data on possible SARS-CoV-2 reinfections from the SI-DEP database.

The upward resumption of the circulation of the coronavirus observed for several weeks in France and the emergence of the Omicron BA.2 sub-lineage, which has been in the majority since the end of February 2022, are “two factors that may play a role in the current trend of increasing numbers of reinfections”according to Public Health France.

Read also: Flu cases on the rise: has our immune system fallen asleep during these two Covid years?

A breakdown of possible cases of reinfection by region and age group

What about the regional distribution of possible cases of reinfection detected in the SI-DEP database between March 2, 2021 and March 20, 2022? “Possible cases of reinfection have been detected in all French regions, with potentially significant differences between regions in terms of numbers and proportion of possible reinfections among all confirmed cases of Covid-19 during of the study period, says the health agency. If in Brittany the proportion of possible cases of reinfection is estimated at 2.6% of all confirmed cases, the Provence-Alpes-Côte D’Azur region reaches a proportion of 5.5%.

Public Health France also notes a higher frequency of reinfection in adults aged 18 to 40 with a proportion of 50% of possible cases than among all confirmed cases of COVID-19 detected in SI-DEP during the study period. Conversely, 1.5% of possible cases of reinfection are aged 80 and over.

Omicron’s fault?

Several recent studies have highlighted the possibility of occurrence of reinfection with the Omicron sub-variant, BA.2, after infection with the BA.1 sub-variant, including within a very short period of time (less than 60 days). However, they agree on the fact that these are probably rare events. In addition, the frequency of reinfections with a result suggestive of Omicron during the two episodes of infections remains very low (less than 1% of cases), as shown in the graph below.

“It is estimated that reinfection with Omicron is around five times higher than with other variants, such as Delta for example. Infection alone, without vaccination, therefore does not appear to confer sufficiently stable immunity over time to completely avoid reinfections,” details virologist Yannick Simonin.

Read also: Covid-19. Are we moving towards an annual vaccination against the virus?

Limits of the study

Be careful however, Public Health France recognizes some limitations in this study to be taken into account in the interpretation that we make of it. First of all, “the analysis of the database could only be made from January 1, 2021 (…) preventing the identification of reinfections that occurred from January 1, 2021 after a first episode of Covid-19 during the year 2020 ». This therefore leads to an underestimation of the frequency of reinfections.

In the same way, “the fact that it is not possible to collect clinical information (for example, absence of symptoms suggestive of Covid-19 between the two episodes), virological (viral load and sequencing) or epidemiological (concept of exposure to risk preceding the positive test) in SI-DEP limits this analysis to possible cases of reinfection”.

Finally, according to the criteria retained for this analysis, there is in particular that of reinfection taken into account if it occurs within a minimum period of 60 days between the two episodes of contamination. Any reinfection that could occur within a shorter period of time is therefore not counted in this study. “Nevertheless, at this stage, we do not have any elements indicating that the occurrence of reinfection with SARS-CoV-2 less than 60 days after a primary infection is a frequent phenomenon, nor that the fact of not taking them into account as part of this analysis could have a significant impact on the interpretation we make of these data in terms of frequency or trend”, concludes the health agency.

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