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

Abstract

Background Understanding the distribution of 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 specimen 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 11,161 households enrolled (5.6%), with 7,483 adults and 1,375 children completing antibody testing. As of June 2021, 33% (95%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, a third of adults and most children in California were seronegative. Serostatus varied regionally and by demographic group.

Association of SARS-CoV-2 BA.4/BA.5 Omicron lineages with immune escape and clinical outcome

“…While the true prevalence of unascertained prior infection in this population is not precisely known, this scenario represents a considerable departure from prior estimates of the reporting fraction in California. 25 It is important to note that our analyses do not distinguish causes of ED presentations and hospital admission, although ED presentations and hospital admissions occurring within 15 days of cases’ first positive test, and ARI-associated hospital admissions, likely have greater specificity for indicating healthcare interactions precipitated by COVID-19 illness. 16,17 Last, our analyses do not distinguish cases infected with BA.4 and BA.5, or cases with BA.2.12.1 versus other BA.2 sublineages, which may be associated with distinct epidemiologic and clinical characteristics.…”

“…The proportion of infections ascertained in our study population is not precisely known; however, this represents a considerable departure from prior estimates of the reporting fraction in California. 18 Whereas bias would require not only the prevalence of prior infection, but the proportion of prior infections ascertained, to differ among cases with BA.4/BA.5 and BA.2 infection, we are unaware of a priori reasons to suspect the latter would occur. It is important to note that our analyses do not distinguish causes of ED presentations and hospital admission, although analyses subset to symptomatic endpoints 19 and our restriction to cases tested in outpatient settings 4 may help to exclude incidentally-identified infections.…”

Evaluating the association between in‐person work and the risk of SARS‐CoV‐2 infection through June 2021

“…Third, although CalScope was designed as a population‐based study with random address‐based sampling, Wave 1 had low participation from racial and ethnic minorities and from households with low income and education levels. While nonrandom participation could have led to collider bias and distorted associations among workers in racial and ethnic minorities and workers with lower household income and education levels, 36 CalScope Wave 1 findings using the entire available data with weighted population estimates showed similar serostatus patterns 13 . Selection bias is also possible given our study excluded individuals experiencing homelessness, severely ill from COVID‐19, or deceased at the time of the study; however, findings are consistent with recent work on COVID‐19 excess mortality in California 7 .…”

“…Sampled households received an invitation in the mail to enroll up to one adult and one child to each complete an online survey and order an antibody test kit. Study protocol and materials were reviewed by the Committee for the Protection of Human Subjects and determined “Exempt.” Full details of the study can be found elsewhere 13 …”

“…Full details of the study can be found elsewhere. 13 We restricted the current study to adults (ages 18+) who completed the online survey and antibody test and self-reported their work status on the date of their survey as one of the following categories (1) employed for wages full-time; (2) employed for wages part-time; (3) self-employed; or (4) out of work for <12 months.…”