Estimated COVID-19 vaccine effectiveness against seroconversion from SARS-CoV-2 Infection, March-October, 2021

Abstract

Background: Monitoring the effectiveness of COVID-19 vaccines against SARS-CoV-2 infections remains important to inform public health responses. Estimation of vaccine effectiveness (VE) against serological evidence of SARS-CoV-2 infection might provide an alternative measure of the benefit of vaccination against infection.

Methods: We estimated mRNA COVID-19 vaccine effectiveness (VE) against development of SARS-CoV-2 anti-nucleocapsid antibodies in March-October 2021, during which the Delta variant became predominant. Participants were enrolled from four participating healthcare systems in the United States, and completed electronic surveys that included vaccination history. Dried blood spot specimens collected on a monthly basis were analyzed for anti-spike antibodies, and, if positive, anti-nucleocapsid antibodies. We used detection of new anti-nucleocapsid antibodies to indicate SARS-CoV-2 infection, and estimated VE by comparing 154 case-participants with new detection of anti-nucleocapsid antibodies to 1,540 seronegative control-participants matched by calendar period. Using conditional logistic regression, we estimated VE ≥ 14 days after the 2nd dose of an mRNA vaccine compared with no receipt of a COVID-19 vaccine dose, adjusting for age group, healthcare worker occupation, urban/suburban/rural residence, healthcare system region, and reported contact with a person testing positive for SARS-CoV-2.

Results: Among individuals who completed a primary series, estimated VE against seroconversion from SARS-CoV-2 infection was 88.8% (95% confidence interval [CI], 79.6%-93.9%) after any mRNA vaccine, 87.8% (95% CI, 75.9%-93.8%) after BioNTech vaccine and 91.7% (95% CI, 75.7%-97.2%) after Moderna vaccine. VE was estimated to be lower ≥ 3 months after dose 2 compared with < 3 months after dose 2, and among participants who were older or had underlying health conditions, although confidence intervals overlapped between subgroups.

Conclusions: VE estimates generated using infection-induced antibodies were consistent with published estimates from clinical trials and observational studies that used virologic tests to confirm infection during the same period. Our findings support recommendations for eligible adults to remain up to date with COVID-19 vaccination.

Keywords: Adults; COVID-19; COVID-19 vaccines; SARS-CoV-2; Serology; Vaccine effectiveness.

Fig. 1

Participants of the COVID-19 Community Research Partnership Study included in analysis of vaccine effectiveness using serologic data. a. Among 29,187 adults enrolled in the study via one of the 4 included health systems in this analysis, 27,535 did not report a previous COVID-19 diagnosis at the time of study enrollment. Of these, 19,235 were sent at least one serology kit between March 1, 2021 and October 31, 2021, and 17,235 returned one or more serology dried blood spot specimens. b. Among 16,675 participants who returned ≥1 serology kits after March 1, 2021, 15,125 returned at least 2 serology kits. c. Among the 676 participants excluded, 625 were excluded because they only received one dose or a non-mRNA vaccine; the remaining 51 participants had an unknown vaccination status. d. Test kit results were excluded based on the timing of vaccination status if a participant was categorized as “partially vaccinated” or having received a third dose at the collection date for the previous negative test.