We use weights obtained through iterative proportional fitting, or raking.11 Raking generates weights by iteratively adjusting the marginal weights of each population control variable (e.g. for population demographics, the prevalence was 2.8% (95% CI 1.3C4.2%), using bootstrap to estimate confidence bounds. These prevalence point estimates imply that 53?000 [95% CI 26?000 to 82?000 using weighted prevalence; 23?000 (95% CI 14?000C35?000) using unweighted prevalence] people were infected in Santa Clara County by late Marchmany more than the 1200 confirmed cases at the time. Conclusion The estimated prevalence of SARS-CoV-2 antibodies in Santa Clara County implies that COVID-19 was likely more widespread than indicated by the number of cases in late March, 2020. At the time, low-burden contexts such as Santa Clara County were far from herd-immunity thresholds. infections. Methods We conducted serologic testing for SARS-CoV-2 antibodies in 3328 adults and children in Santa Clara County using capillary blood draws and a lateral-flow immunoassay. In this section, we describe our sampling and recruitment approaches, specimen-collection methods, antibody-testing procedure, test-kit validation and statistical methods. Our protocol was informed by a World Health Organization protocol for population-level COVID-19 antibody testing.8 We conducted our study with the cooperation of the Santa Clara County Department of Public Health. Study participants and sample recruitment We recruited participants by placing targeted advertisements on Facebook aimed at residents of Santa Clara County. We used Facebook to quickly reach a large number of county residents and because it allows granular targeting by zip code and socio-demographic characteristics.9 We posted our advertisements targeting two populations: ads aimed at a representative population of the county by zip code and specially targeted ads Dehydroepiandrosterone to balance our sample for under-represented zip codes. In addition, we capped Dehydroepiandrosterone registrations from overrepresented areas after our registration slots filled up quickly with participants from wealthier zip codes. Individuals who clicked on the advertisement were directed to a survey hosted by the Stanford REDCap platform, which provided information about the study.10 The survey asked for six data elements: zip code of residence, age, sex, race/ethnicity, underlying co-morbidities and prior clinical symptoms. Over 24?hours, we registered 3285 adults, and each adult was allowed to bring one child from the same household with them (889 children registered). Additional details of the participant-selection process are provided below (and in Supplementary Data, available as Supplementary data at online). Specimen-collection and testing methods We established drive-through test sites in three locations spaced across Santa Clara County: two county parks in Los Gatos and San Jose, and a church in Mountain View. Only individuals with a participant identification (participant ID) were allowed into the testing area. Verbal informed consent was obtained Dehydroepiandrosterone to minimize participant and staff exposure. With participants in their vehicles, sample collectors in personal protective equipment drew 50C200L of capillary blood into an EDTA-coated microtainer. Tubes were barcoded and linked with the Rabbit Polyclonal to VEGFB participant ID. Samples were couriered from the collection sites to a test-reading facility with steady lighting and climate conditions. Technicians drew whole blood up to a fill line on the manufacturers pipette and placed it in the test-kit well, followed by a buffer. Test kits were read 12C20?minutes after the buffer was placed. Technicians barcoded tests to match sample barcodes and Dehydroepiandrosterone documented all test results. Test kit performance The manufacturers performance characteristics were available prior to the study (using 85 Dehydroepiandrosterone confirmed positive and 371 confirmed negative samples). We conducted additional testing to assess the kit performance and continued collecting information from assessments of test performance to incorporate into the analysis..