mRNA Vaccine Effectiveness against COVID-19 among Symptomatic Outpatients Aged ≥16 Years in the United States, February – May 2021

Abstract

Evaluations of vaccine effectiveness (VE) are important to monitor as COVID-19 vaccines are introduced in the general population. Research staff enrolled symptomatic participants seeking outpatient medical care for COVID-19-like illness or SARS-CoV-2 testing from a multisite network. VE was evaluated using the test-negative design. Among 236 SARS-CoV-2 nucleic acid amplification test-positive and 576 test-negative participants aged ≥16 years, VE of mRNA vaccines against COVID-19 was 91% (95% CI: 83-95) for full vaccination and 75% (95% CI: 55-87) for partial vaccination. Vaccination was associated with prevention of most COVID-19 cases among people seeking outpatient medical care.

Vaccine effectiveness against COVID‐19 among symptomatic persons aged ≥12 years with reported contact with COVID‐19 cases, February–September 2021

“…This analysis includes four additional months of data compared with an earlier evaluation of COVID‐19 VE between February and May 2021. 3 Despite predominance of the Delta variant in the latter study period, 7 our findings show protection against laboratory‐confirmed symptomatic illness. A decline in VE point estimates in the latter period could be attributed to reduced protection against the Delta or a result of waning protection of the initial vaccination series.…”

“…Methods used for estimating VE against laboratory‐confirmed symptomatic SARS‐CoV‐2 infection or COVID‐19 among persons seeking medical care/testing for SARS‐CoV‐2 in Flu VE Network study sites have been previously described. 3 Briefly, research staff screened persons who sought outpatient medical care (i.e., telehealth, primary care, urgent care, and/or emergency department) or clinical SARS‐CoV‐2 testing using a standard case‐definition for COVID‐like illness that included onset of fever/feverishness, cough, or loss of taste/smell with symptom duration <10 days. 4 Research staff contacted potentially eligible outpatients in person, by telephone, or email to confirm eligibility and enroll participants who consented verbally or in writing.…”

“…We compared VE using plausible self‐report of vaccination to VE using only documented vaccination status, where plausibility was determined by ability to report credible location of vaccination, as previously described. 3 We also compared findings when persons who identified as working in healthcare were excluded and also when persons with unknown status were classified as having had no known contact. Statistical analyses were conducted using SAS version 9.4 (SAS Institute Inc., Cary, NC, USA).…”

“…An interaction term for known contact and vaccination status was assessed. Models were adjusted as previously described and included age, study site, enrollment period, and self‐reported race/ethnicity 3 . We performed stratified analyses by (1) time of illness onset using February–May 2021 as a pre‐Delta variant period and July—September 2021 as the Delta variant‐predominant period and (2) by age group (as 12–49 years and >49 years due to sample size) 7…”

Effectiveness of Covid-19 vaccines against symptomatic and asymptomatic SARS-CoV-2 infections in an urgent care setting

“…Misclassification of exposure is a possibility as vaccination status was self-reported and dates of vaccination were not collected. However, other studies have shown very high agreement between self-reported and recorded COVID-19 vaccination status 67,68 . VE after one dose of mRNA vaccines is likely underestimated in this study because it was not known if the outcome occurred more than 15 days after the first dose.…”