In late 2019, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) emerged in China and has since caused a pandemic of Coronavirus Disease 2019 (COVID-19). The first COVID-19 case in New York City (NYC) was officially confirmed on March 1st 2020 followed by a severe local epidemic.

To understand seroprevalence dynamics, we conducted a retrospective, repeated cross-sectional analysis of anti-SARS-CoV-2 spike antibodies in weekly intervals from the beginning of February to July 2020 using more than 10,000 plasma samples from patients at Mount Sinai Hospital in NYC. Here we show the dynamics of seroprevalence in an ‘urgent care’ (UC) group, enriched for COVID-19 cases during the epidemic, and a ‘routine care’ group (RC), which more closely represents the general population. Seroprevalence increased at different rates in both groups, with seropositive samples as early as mid-February, and levelled out at slightly above 20% in both groups after the epidemic wave subsided by the end of May. From May to July seroprevalence stayed stable, suggesting lasting antibody levels in the population. Our data suggest an earlier than previously documented introduction of SARS-CoV-2 into NYC and describe the dynamics of seroconversion over the full course of the first pandemic wave in a major metropolitan area.

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