CalScope: Monitoring SARS-CoV-2 Seroprevalence from Vaccination and Prior Infection in Adults and Children in California May 2021– July 2021

ABSTRACT

Background Understanding the distribution of SARS-CoV-2 antibodies from vaccination and/or prior infection is critical to the public health response to the pandemic. CalScope is a population-based serosurvey in 7 counties in California. Methods We invited 200,000 randomly sampled households to enroll up to 1 adult and 1 child between April 20, 2021 and June 16, 2021. We tested all specimen for antibodies against SARS-CoV-2 nucleocapsid and spike proteins, and each participant completed an online survey. We classified participants into categories seronegative, antibodies from infection only, antibodies from infection and vaccination, and antibodies from vaccination only. Results 11,161 households enrolled (5.6%), with 7,483 adults and 1,375 children completing antibody testing. As of June 2021, 33% (95%CI [28%, 37%]) of adults and 57% (95%CI[48%, 66%]) of children were seronegative;18% (95%CI[14%, 22%]) of adults and 26% (95%CI[19%, 32%]) of children had antibodies from infection alone;9% (95%CI[6%,11%]) of adults and 5% (95%CI[1%, 8%]) of children had antibodies from infection and vaccination;and 41% (95%CI[37%, 45%]) of adults and 13% (95%CI [7%, 18%]) of children had antibodies from vaccination alone. Conclusions As of June 2021, a third of adults and most children in California were seronegative. Serostatus varied regionally and by demographic group.