Addressing COVID-19 is a pressing health and social concern.To date, many epidemic projections and policies addressing COVID-19 have been designed without seroprevalence data to inform epidemic parameters.
We measured the seroprevalence of antibodies to SARS-CoV-2 in Santa Clara County.Methods On 4/3-4/4, 2020, we tested county residents for antibodies to SARS-CoV-2 using a lateral flow immunoassay.Participants were recruited using Facebook adstargeting a representative sample of the county by demographic and geographic characteristics.We report the prevalence of antibodies to SARS-CoV-2 ina sample of 3,330 people, adjusting for zip code, sex, and race/ethnicity.We also adjust for test performance characteristics using 3 different estimates: (i) the test manufacturer’s data, (ii) a sample of 37 positive and 30 negative controls tested at Stanford, and (iii) a combination of both.
Theunadjusted prevalence of antibodies to SARS-CoV-2 in Santa Clara County was 1.5% (exact binomial 95CI 1.11-1.97%), andthe population-weighted prevalence was 2.81% (95CI 2.24-3.37%).
Under the three scenarios for test performance characteristics, the population prevalence of COVID-19 in Santa Clara rangedfrom 2.49% (95CI 1.80-3.17%) to 4.16% (2.58-5.70%).
These prevalence estimates represent a range between 48,000 and 81,000 people infected in Santa Clara County by early April,50-85-fold more than the number of confirmed cases.
The population prevalence of SARS-CoV-2 antibodies in Santa Clara County implies that the infection is much more widespread than indicated by the number of confirmed cases.
Population prevalence estimates can now be used to calibrate epidemic and mortality projections.
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