CalScope: Monitoring Severe Acute Respiratory Syndrome Coronavirus 2 Seroprevalence From Vaccination and Prior Infection in Adults and Children in California May 2021-July 2021.

Background: Understanding the distribution of severe acute respiratory syndrome coronavirus 2 (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 specimens 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: A total of 11 161 households enrolled (5.6%), with 7483 adults and 1375 children completing antibody testing. As of June 2021, 33% (95% confidence interval [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, one third of adults and most children in California were seronegative. Serostatus varied regionally and by demographic group.