why do we not know the number of infected since the start of the epidemic?

Since the beginning of the year, about one in five French people has been recognized as positive for SARS-CoV-2, according to data collected on the SI-Dep database, which since May 2020 has listed the results of virological tests. . How many people have received Covid-19 since the start of the epidemic in France more than two years ago? “There will never be raw data” in this regard, warns epidemiologist and model builder Mircea Sofonea from the University of Montpellier.

If we add the number of people who have been declared positive in the indicators from the SI-Dep database, we reach almost 24 million cases. We can add almost 140,000 infections identified by the health authorities before the production of this file (since the patients with Covid-19 were identified in a rather traditional way).

But this calculation is very imperfect for several reasons:

  • Massive late screening. In the first months of the epidemic, virological tests were reserved for the most serious patients. According to estimates by Public Health France (SPF), only one in twenty-four infections was registered from March to May 2020. It was not until the summer after the first wave that the screening was extended.
  • A changing screening policy and offers. The detection rate of the virus varies greatly over time. The reasons may be personal: the French have thus been tested more during the holidays when reunited with their families. But it is above all the screening policy that has changed many times over these two years. The tests were first exclusively PCR, then antigenic, available on prescription and then free, with appointments more or less easy to get depending on menstruation, etc. From 15 October 2021, reimbursement of so-called screenings will cease. “of comfort” for unvaccinated adults have e.g. led to a decrease in screening in unvaccinated and asymptomatic adults, and infections have gone undetected.
  • Contaminated people have never been tested. “Not all infected people are tested, either because they are not symptomatic and do not think about it, or because they do not have access to the test for various reasons”summed up Clara Blondel, scientific project manager, during the epidemiological update organized by the SPF on 11 March.
  • Self-tests do not count. Since the end of 2021, the use of self-tests has become widespread, especially among children, but these screenings are not counted in SI-Dept. In theory, a positive result should be confirmed by an antigen test or RT-PCR, but there is no guarantee that this rule will be observed. Not to mention that the reliability of the results is not without limits.
  • Reinfections spoke several times. People who on several occasions have been so unlucky to get Covid-19 have been repeatedly mentioned in the indicators provided by the health authorities. The SPF identified 417,000 people who had at least two positive tests more than two months apart between 1.eh January 2021 and January 27, 2022. However, due to the absence of a specific information system before May 2020 and changes in the data pseudonymisation phase of it in early 2021, this census does not include “possible re-infection events with a first episode in 2020”, SPF emphasizes in a recent publication. There are no data available on reinfections that have occurred since this publication. By comparison, UK health authorities have detected 800,000 possible re-infections out of 17.1 million confirmed cases since the end of January 2020.
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Other methods of completing the official balance sheet

From the start of the epidemic, several teams of researchers wanted to estimate the number of people with Covid-19 using methods other than virological tests. It was especially about “to answer a question: have we gained collective immunity?”explains Mircea Sofonea.

  • First estimate based on hospital data

In France, from the beginning of the epidemic, researchers from the Institut Pasteur, Public Health France and Inserm combined French hospital data and the study conducted on board Diamond Princess, a cruise ship quarantined in early 2020, to determine hospitalization and deaths for Covid-19. According to this extrapolation, approximately 3.5 million people (between 2.1 million and 6 million) would have been affected by 11 May 2020, according to their work published in the journal Science May 13, 2020.

  • The limits of serological examinations

The advent of seroprevalence studies, which search in blood samples for the presence of antibodies directed against SARS-CoV-2, has provided a different view. However, the relationship between the presence of antibodies and infection with SARS-CoV-2 has been gradually modified by vaccination – antibodies directed against the nail protein (S) can be distinguished, which may also be the result of an infection other than vaccination and those directed against the nucleocapsid protein (N ), which is solely the result of an infection, but which could be detected in a shorter time than the former.

In any case, the results of these studies should be carefully analyzed: “Antibodies are known to decrease over time, so it is difficult to interpret seroprevalence data”, emphasizes Simon Cauchemez, head of the mathematical modeling unit for infectious diseases at the Institut Pasteur. Moreover, not all cases “of seroconversion, that is, the fact of producing antibodies”Mr. Sofonea abounds.

But serological studies have enabled researchers at the Institut Pasteur to develop a new method: it is about determining the likelihood of being admitted by infection. “by comparing the number of admissions during the first wave with the number of infections estimated by the large Sapris seroprevalence study during this wave.

By knowing the number of hospitalized people, it thus becomes possible “to rebuild the number of infected” over time, details Mr. Cauchemez. But this method had to stop being used “with the emergence of new, more serious variants and with vaccination of the population, because these two factors change the probability (…) hospitalization we need ”explains Mr Cauchemez.

  • “Between 65% and 70%” of the French polluted at least once

Several modeling works are underway. At the Institut Pasteur, Mr Cauchemez explains that the teams are working on it “to develop a more complex approach, which will make it possible both to estimate the number of French people who have been infected, but also to quantify the different immunity profiles; [par exemple vacciné et infecté, etc.] ». But this work is “quite complicated”.

Mr. Sofonea’s approach is based on hospital mortality, “the most reliable data available”. Then you must “going back to the number of infected cases: it’s more complicated because you have to use the probability of dying while you are infected, which varies” over time due to several factors, such as the variant in question, improvement of treatment techniques or increase in vaccination power. While the method is still being improved, his team believes so “between 65% and 70%” the number of French people infected at least once since the start of the epidemic.

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