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Seroprevalence of Antibodies to SARS-CoV-2
Quick Takes
- Seropositivity for SARS-CoV-2 varied from 1.0-6.9% in 10 different sites across the United States.
- Extrapolation to site population suggests exposure to SARS-CoV-2 is 6-24 times higher than reported cases.
- Mild or asymptomatic SARS-CoV-2 infections are likely much more common than previously thought.
Study Questions:
What is the estimated seropositivity for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in 10 sites across the United States?
Methods:
Residual sera were collected from March 23–May 12, 2020 from 16,025 persons undergoing routine clinical testing in the San Francisco Bay area; Connecticut; south Florida; Louisiana; Minneapolis-St. Paul-St. Cloud metro area; Missouri; New York City metro area; Philadelphia metro area; Utah; and western Washington State. The timeline of sera collection varied across sites. The sera were tested at the Centers for Disease Control and Prevention with a screening assay followed by a confirmatory one for SARS-CoV-2 spike protein. Specificity of the assay was 99.3% (95% confidence interval [CI], 98.3%-99.9%) and sensitivity was 96.0% (95% CI, 90.0%-98.9%). The number of infections at each site was estimated by extrapolating seroprevalence to site populations, and then compared to the number of coronavirus disease 2019 (COVID-19) cases reported during that same time period.
Results:
Over half of people tested (55.2%) were women; 1,205 (7.5%) were ≤18 years and 5,845 (36.2%) were ≥65 years. The proportion of seropositivity to SARS-CoV-2 was overall low, ranging from 1.0% in the San Francisco Bay area (collected April 23-27) to 6.9% in New York City (collected March 23-April 1). There was no clear association between seroprevalence by age and sex across sites. The estimated number of infections ranged from 6-24 times the number of reported cases; for seven sites (Connecticut, Florida, Louisiana, Missouri, New York City metro area, Utah, and western Washington State), an estimated >10 times more SARS-CoV-2 infections occurred than the number of reported cases.
Conclusions:
Seropositivity for SARS-CoV-2 varied from 1.0%-6.9% in 10 different sites across the United States.
Perspective:
This study is the first estimating seroprevalence across multiple sites in the United States. Estimates in some cases differ greatly from other seroprevalence studies. Study limitations such as the lack of random selection and subsequent high risk of selection bias makes accurate estimates challenging. There are also many unknowns that preclude deriving strong conclusions from these studies; such as the timeline of development of antibody to SARS-CoV-2 or whether it occurs at all in every exposed individual and the potential cross-reactivity with common coronavirus yielding false-positive testing. The one unifying conclusion from the seroprevalence studies is that exposure to SARS-CoV-2 is likely much higher than reported cases, suggesting that mild or asymptomatic SARS-CoV-2 infections are much more common than previously thought.
Clinical Topics: COVID-19 Hub, Prevention
Keywords: Antibodies, Coronavirus, COVID-19, Diagnostic Tests, Routine, False Positive Reactions, Primary Prevention, SARS Virus, severe acute respiratory syndrome coronavirus 2, Seroepidemiologic Studies
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