CalScope: Monitoring SARS-CoV-2 Seroprevalence from Vaccination and Prior Infection in Adults and Children in California May 2021– July 2021
Authors: Mehrotra Megha L. , Lim Esther , Lamba Katherine , Kamali Amanda , Lai Kristina W. , Meza Erika , Szeto Irvin , Robinson Peter , Tsai Cheng-ting , Gebhart David , Fonseca Noemi , Martin Andrew B. , Ley Catherine , Scherf Steve , Watt James , Seftel David , Parsonnet Julie , Jain Seema 
Published: December 2021

Importance

Understanding how SARS-CoV-2 seroprevalence varies regionally across California is critical to the public health response to the pandemic.

Objective

To estimate how many Californians have antibodies against SARS-CoV-2 from prior infection or vaccination.

Design

Wave 1 of CalScope: a repeated cross-sectional serosurvey of adults and children enrolled between April 20, 2021 and June 16, 2021.

Setting

A population-based random sample of households in seven counties in California (Alameda, El Dorado, Kern, Los Angeles, Monterey, San Diego, and Shasta) were invited to complete an at-home SARS-CoV-2 antibody test and survey instrument.

Participants

Invitations were sent to 200,000 randomly selected households in the seven counties. From each household, 1 adult (18 years and older) and 1 child (aged 6 months to 17 years) could enroll in the study. There were no exclusion criteria.

Main Outcome(s) and Measures

All specimens were tested for antibodies against the nucleocapsid and spike proteins of SARS-CoV-2. The primary outcome was serostatus category, which was determined based on antibody test results and self-reported vaccination status: seronegative, antibodies from infection only, antibodies from infection and vaccination, and antibodies from vaccination alone. We used inverse probability of selection weights and iterative proportional fitting to account for non-response.

Results

11,161 households enrolled in wave 1 of CalScope, with 7,483 adults and 1,375 children completing antibody testing. As of June 2021, 27% (95%CI [23%, 31%]) of adults and 30% (95%CI [24%, 36%]) of children had evidence of prior SARS-CoV-2 infection; 33% (95%CI [28%, 37%]) of adults and 57% (95%CI [48%, 66%]) of children were seronegative. Serostatus varied regionally. Californians 65 years or older were most likely to have antibodies from vaccine alone (59%; 95%CI [48%, 69%]) and children between 5-11 years old were most likely to have antibodies from prior infection alone (36%; 95%CI [21%, 52%]).

Conclusions and Relevance

As of June 2021, a third of adults in California and most children under 18 remained seronegative. Seroprevalence varied regionally and by demographic group, suggesting that some regions or populations might remain more vulnerable to subsequent surges than others.

Key Points

Question

What is the prevalence of vaccine and infection derived antibodies against SARS-CoV-2 in adults and children in California?

Findings

In this population-based serosurvey that included 11,161 households, as of June 2021, 33% of adults and 57% of children were seronegative; 18% of adults and 26% of children had antibodies from infection alone; 9% of adults and 5% of children had antibodies from both infection and vaccination; and 41% of adults and 13% of children had antibodies from vaccination alone.

Meaning

Serostatus varied considerably across geographic regions, suggesting that certain areas might be at increased risk for future COVID-19 surges.