Since December 2021 and the expansion of the Omicron variant in France, the frequency of possible reinfections by covid-19 has increased remarkably. 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 since the end of summer 2020, “We know that for some people the protection conferred by the disease is not perfect, it is not complete and therefore one can get sick a second time with more modest symptoms than the first”, explains Matthieu Revest, an infectious disease specialist at Rennes University Hospital (Ille-et-Vilaine).
After briefly stabilizing in early January 2022, the ratio of reported possible reinfection cases to total Covid cases has increased again since the end of January and now represents 5.4% of all confirmed cases, according to the latest data. available data. published on friday 1is 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, the health agency said during its latest weekly news conference.
A reinfection after a 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 fairly rapidly over time.”.
The Omicron variant tends to induce less severe forms of the disease but more reinfections. In practice, “Some people who have recently contracted Omicron (mainly the BA.1 subvariant) may be infected again one to two months later, in particular by the BA.2 subvariant, now the majority in France, continues the specialist in emerging viruses. Although reinfection after one month is not 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 subvariant. »
How to explain this phenomenon of reinfection?
In total, 685,858 possible cases of reinfection were identified between March 2, 2021 and March 20, 2022, including 95.2% since December 6, 2021, marking the beginning of the spread of the variant Omicron in France, reports the health agency in its latest weekly statement. “It seems likely that the attenuation of the post-vaccination or post-infectious immune response in 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 received a booster dose of the vaccine,” indicates Public Health France. And to add that“It is also very likely that the strong distribution in France of the Omicron variant, characterized by increased transmissibility and significant immune escape, amplifies this phenomenon.”.
88% of potential reinfection cases for which an interpretable screening result was available for the reinfection episode also had an evocative result of Omicronaccording 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 appearance of the Omicron BA.2 sublineage, 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.
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 the number and proportion of possible reinfections among all confirmed cases of Covid-19 during 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 observes a higher frequency of reinfection in adults aged 18 to 40 years with a proportion of 50% of possible cases than among all confirmed cases of COVID-19 detected in SI-DEP during the study period. In contrast, 1.5% of potential reinfection cases are 80 years of age or older.
Several recent studies have highlighted the possibility of reinfection with the Omicron subvariant, BA.2, after infection with the subvariant BA.1, even in a very short period of time (less than 60 days). However, they do agree 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 about five times higher than with other variants, such as Delta for example. Therefore, infection alone, without vaccination, does not seem to confer sufficiently stable immunity over time to completely prevent reinfections.” virologist Yannick Simonin details.
However, be careful, Public Health France recognizes some limitations in this study that must be taken into account in our interpretation of it. First of all, “The analysis of the database could only be carried out as of January 1, 2021 (…) preventing the identification of reinfections that occurred as of January 1, 2021 after a first episode of Covid-19 during the year 2020”. Therefore, this 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 risk exposure prior to the positive test) in SI-DEP limits this analysis to possible cases of reinfection”.
Finally, according to the criteria used for this analysis, reinfection is taken into account in particular if it occurs within a minimum period of 60 days between the two contamination episodes. Any reinfection that might occur in a shorter period of time is therefore not counted in this study. “However, at this stage we do not have elements that indicate that the occurrence of reinfections by 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 how we interpret these data in terms of frequency or trend.” concludes the health agency.