Seroprevalence of SARS-CoV-2 Among Adults in Three Regions of France Following the Lockdown and Associated Risk Factors: A Multicohort Study

Background: To estimate the seroprevalence of SARS-CoV-2 infection in May-June 2020 after the lockdown in adults living in three regions in France and to identify the associated risk factors. Methods: Participants in a survey on COVID-19 from an existing consortia of three general adult population cohorts living in the Ile-de-France (IDF) or Grand Est (GE) - two regions with high rate of COVID-19, or in the Nouvelle-Aquitaine (NA) – with a low rate, were asked to take a dried-blood spot (DBS) for anti-SARS-CoV-2 antibodies.The primary outcome was a positive anti-SARS-CoV-2 ELISA IgG result against the spike protein of the virus (ELISA-S). The secondary outcomes were a positive ELISA IgG against the nucleocapsid protein (ELISA-NP), anti-SARS-CoV-2 neutralizing antibodies titers ≥40 (SN), and predicted positivity obtained from a multiple imputation model (MI). Prevalence estimates were adjusted using sampling weights and post-stratification methods. Findings: Between May 4, 2020 and June 23, 2020, 16,000 participants were asked to provide DBS, and 14,628 were included in the analysis, 983 with a positive ELISA-S, 511 with a positive ELISA-NP, 424 with SN≥40 and 941±31 with a positive MI. Adjusted estimates of seroprevalence (positive ELISA-S) were 10.0% (95%CI 9.1%;10.9%) in IDF, 9.0% (95%CI 7.7%; 10.2%) in GE and 3.1% (95%CI 2.4%; 3.7%), in NA. The adjusted prevalence of positive ELISA-NP, SN and MI were 5.7%, 5.0% and 10.0% in IDF, 6.0%, 4.3% and 8.6% in GE, and 0.6%, 1.3% and 2.5% in NA, respectively. A higher seroprevalence was observed in younger participants and when at least one child or adolescent lived in the same household. A lower seroprevalence was observed in smokers compared to non-smokers. Interpretation: At the end of the lockdown the prevalence of anti-SARS-CoV-2 IgG or neutralizing antibodies remained low in the French adult population, even in regions with high reported rates of COVID-19. Funding Statement: ANR (Agence Nationale de la Recherche, #ANR-20-COVI-000, #ANR-10-COHO-06), Fondation pour la Recherche Medicale (#20RR052-00), Inserm (Institut National de la Sante et de la Recherche Medicale, #C20-26). Additional cohort funding is listed in the manuscript. Declaration of Interests: Prof Fabrice Carrat reports grants from INSERM-ANRS, during the conduct of the study; personal fees from Imaxio, outside the submitted work. Mathilde Touvier, Prof Gianluca Severi, Prof Laurence Meyer, Prof Florence Jusot, Nathanael Lapidus, Delphine Rahib, Nathalie Lydie, Helene Blanche, Jean-Francois Deleuze, Marie-Aline Charles, Prof Pierre-Yves Ancel, Alexandra Rouquette, Claude Martin, Prof Xavier de Lamballerie, Prof Marie Zins, Nathalie Bajos declare no competing interest. The remaining authors have no competing interest to declare. Ethics Approval Statement: Ethical approval and written or electronic informed consent were obtained from each participant before enrolment in the original cohort. The SAPRIS survey was approved by the Inserm ethics committee (approval #20-672 dated March 30, 2020). The SAPRIS-SERO study was approved by the Sud-Mediterranee III ethics committee (approval #20.04.22.74247) and electronic informed consent was obtained from all participants for DBS testing.