Seroconversion of a city: Longitudinal monitoring of SARS-CoV-2 seroprevalence in New York City

By conducting a retrospective, cross-sectional analysis of SARS-CoV-2 seroprevalence in a sentinel group (enriched for SARS-CoV-2 infections) and a screening group (representative of the general population) using >5,000 plasma samples from patients at Mount Sinai Hospital in New York City (NYC), we identified seropositive samples as early as in the week ending February 23, 2020. A stark increase in seropositivity in the sentinel group started the week ending March 22 and in the screening group in the week ending March 29. By the week ending April 19, the seroprevalence in the screening group reached 19.3%, which is well below the estimated 67% needed to achieve community immunity to SARS-CoV-2. These data potentially suggest an earlier than previously documented introduction of SARS-CoV-2 into the NYC metropolitan area.