Cumulative incidence of sars-cov-2 infection and epidemic metrics, united states

ABSTRACT

Background: Understanding the cumulative incidence of SARS-CoV-2 infections in the United States has been limited by asymptomatic infections, waning antibodies after natural infection, incomplete case ascertainment and reporting, and limited representative samples. We conducted a probability survey of US households to measure SARS-CoV-2 infection and immune response and to estimate the cumulative incidence of SARS-CoV-2 infection. Methods: A multistage random sample of US postal addresses were mailed a kit to self-collect an anterior nares swab and a dried blood spot (DBS) sample from August to December 2020. Specimens were tested by EUA-approved PCR and serology tests. Weighted estimates of antibody prevalence, together with historical patterns of antibody waning, were used to estimate the cumulative incidence of SARS-CoV-2 infections, the diagnosed fraction, and infection fatality ratio (IFR). Weighted estimates were used to calculate prevalence ratios comparing demographic, geographic, and clinical subgroups. Results: 37,056 kits were mailed to sampled US households. Overall, 5,666 surveys were completed by December 8, 2020;of these, 4,654 also returned a DBS specimen with a valid antibody result. Overall participation rate was 11.8%. We estimated 39,421,841 (95% credible interval (CrI) 33,759,801-43,958,068) total infections by October 30, 2020, an estimated diagnosed fraction of 17% (95% Crl 15-21%) and an estimated IFR of 0.64% (95% CrI 0.58-0.75%). Daily seroprevalence peaked by Sept 2020 and remained stable through November 2020 due to a balance of waning antibodies and new infections (Figure). Non- Hispanic Black (PR 2.2;95% confidence interval (CI) 1.2-4.0) and Hispanic (PR 3.1, CI 1.8-5.3) respondents were more likely than White non-Hispanic to have laboratory evidence of prior SARS-CoV-2 infection. Prevalence was also higher among those living in metropolitan areas (PR vs non-metropolitan areas 2.5, CI 1.3-5.0) and among those reporting cold or flu symptoms (PR 2.6, CI 1.6-4.1) or loss of taste or smell (PR 12.8, CI 8.5-19.4) since January 1, 2020. Conclusion: We report the results of the first national probability sample of US households to assess the prevalence of antibodies to SARS-CoV-2 and cumulative incidence. As of October 30, 2020, about 1 in 8 US residents aged ≥18 years had been infected with SARS-CoV-2, and about 1 in 6 of those had been diagnosed. Household-based probability surveys provide a minimally biased benchmark to characterize epidemic dynamics.