Monitoring the proportion infected by SARS-CoV-2 from age-stratified hospitalisation and serological data

BackgroundRegional monitoring of the proportion infected by SARS-CoV-2 is important to guide local management of the epidemic, but is difficult in the absence of regular nationwide serosurveys.MethodsWe developed a method to reconstruct in real-time the proportion infected by SARS-CoV-2 and the proportion of infections being detected from the joint analysis of age-stratified seroprevalence, hospitalisation and case data. We applied our approach to the 13 French metropolitan regions.FindingsWe estimate that 5.7% [5.1%-6.4%] of adults in metropolitan France had been infected by SARS-CoV-2 by May 2020. This proportion remained stable until August and increased to 12.6% [11.2%-14.3%] by the end of November. With 23.8% [21.2%-26.8%] infected in the Paris region compared to 4.0% [3.5% - 4.6%] in Brittany, regional variations remained large (Coefficient of Variation CV: 0.53) although less so than in May (CV: 0.74). The proportion infected was twice higher (17.6% [13.4%-22.7%]) in 20-49 y.o. than in 50+ y.o (8.0% [5.7% - 11.5%]). Forty percent [33.7% - 45.4%] of infections in adults were detected in June-August compared to 55.7% [48.7% - 63.1%] in September-November. Our method correctly predicted seroprevalence in 11 regions in which only hospitalisation data were used.InterpretationIn the absence of contemporary serosurvey, our real-time monitoring indicates that the proportion infected by SARS-CoV-2 may be above 20% in some French regions.FundingEU RECOVER, ANR, Fondation pour la Recherche Médicale, Inserm.