ABSTRACT
Background Although the COVID-19 pandemic peaked in March/April 2020 in France, the prevalence of infection is barely known. Herein, we assessed the serological response against the SARS-CoV-2 virus in a large population working in one institution of the Paris conurbation. We set up two high-throughput and sensitive methods to assess SARS CoV-2 Nucleoprotein and Spike protein-specific IgG response along with a pseudo-neutralization assay in sera. We studied 1847 participants who also answered a web-based survey on clinical symptoms. Methods and Results In May-July 2020, 11% (95% CI: 9.7-12.6) of serums were positive for IgG against the SARS-CoV-2 N and S protein and 9.5% (CI:8.2-11.0) were pseudo-neutralizer. The prevalence of immunization was 11.6% (CI:10.2-13.2) considering positivity in at least one assays. In 5% (CI:3.9-7.1) of RT-qPCR positive individuals, no systemic IgGs were detected. Among immune individuals, 21% had been asymptomatic. Anosmia and ageusia occurred in 52% of the IgG-positive individuals and in 3% of the negative ones. In contrast, 30% of the anosmia-ageusia cases were seronegative suggesting that the true prevalence of infection may reach 16.6%. In sera obtained 4-8 weeks after the first sampling anti-N and anti-S IgG titers and pseudo-neutralization activity declined by 31%, 17% and 53%, respectively with half-life of 35, 87 and 28 days, respectively. Conclusions The population studied being not particularly exposed to SARS-CoV-2 infection is representative of active workers in the Paris conurbation, suggesting that the current epidemiological models may underestimate the true prevalence of infection. The short lifespan of the serological systemic responses hinders retrospective assessment of the epidemic extent.