The study shows that transmission remained low until early March 2020, with an overall infection prevalence of less than 1%. The rate of infection then rose sharply in March 2020, reaching a prevalence of approximately 4% of the population by early April. Our third measurement, at approximately 5%—corresponding to the cumulative number of infections through early May 2020—shows the slowing effect caused by the first lockdown.

The risk of infection does not appear to differ by gender, and we observe a relatively consistent prevalence across age groups, with the exception of children under ten, who are significantly less affected than the rest of the population. This finding among children confirms that young children are less susceptible to or less likely to transmit SARS-CoV-2 than they are to pandemic influenza, for example.

Throughout this first wave of the epidemic in 2020, the hardest-hit regions were Île-de-France and the Grand-Est region, where the first cases and major clusters were detected in mainland France. We also observed a more unexpected finding with a high prevalence rate in French Guiana as early as May 2020, whereas other surveillance indicators such as hospitalizations or deaths from COVID-19 did not show a visible increase until July 2020. We attribute this to the region’s particularly young age structure. Since young people are at lower risk of developing severe forms of the disease, a higher prevalence is required before the consequences of the disease begin to show up in hospital indicators.

Estimating the proportion of the population that has been infected since the start of the epidemic allows us to calculate case fatality rates by dividing the number of deaths recorded during the relevant periods by the number of infected individuals. Similarly, we can calculate the risk of hospitalization for an infected individual. In both cases, we observed a risk that increases exponentially with age.

We also estimated that only 1 in 24 infections was recorded as a confirmed case by epidemiological surveillance during the study period (March to May 2020). This discrepancy is explained by the restrictive case definition, limited access to diagnostic testing, and the absence of systematic screening during this early part of 2020.

Finally, in the absence of a vaccine during 2020, it was crucial to quantify the proportion of the population potentially immune due to prior infection. We estimated that approximately 2.3 million people infected through May 2020 had a neutralizing response indicating potential protection against the virus¹. This figure is, of course, insufficient to expect herd immunity and prevent the virus from continuing to spread after this first wave.

Serological surveillance continued after the first wave, with two rounds of testing conducted during the second wave of the epidemic. The results were reported in the weekly epidemiological update and showed a slow increase in prevalence to 9% in October 2020 and 13% in February 2021 (see all of our epidemiological updates). We plan to continue periodically measuring the prevalence and immunity levels² of the French population in 2021, taking into account the ramp-up of vaccination, the potential decline in antibodies, and the diversity of circulating strains.