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

Abstract

Background Individuals in contact with persons with COVID‐19 are at high risk of developing COVID‐19; protection offered by COVID‐19 vaccines in the context of known exposure is poorly understood. Methods Symptomatic outpatients aged ≥12 years reporting acute onset of COVID‐19‐like illness and tested for SARS‐CoV‐2 between February 1 and September 30, 2021 were enrolled. Participants were stratified by self‐report of having known contact with a COVID‐19 case in the 14 days prior to illness onset. Vaccine effectiveness was evaluated using the test‐negative study design and multivariable logistic regression. Results Among 2229 participants, 283/451 (63%) of those reporting contact and 331/1778 (19%) without known contact tested SARS‐CoV‐2‐positive. Adjusted vaccine effectiveness was 71% (95% confidence interval [CI], 49%–83%) among fully vaccinated participants reporting a known contact versus 80% (95% CI, 72%–86%) among those with no known contact ( p ‐value for interaction = 0.2). Conclusions This study contributes to growing evidence of the benefits of vaccinations in preventing COVID‐19 and support vaccination recommendations and the importance of efforts to increase vaccination coverage.

Effectiveness of two and three mRNA COVID‐19 vaccine doses against Omicron‐ and Delta‐Related outpatient illness among adults, October 2021–February 2022

“…Second, participants who reported a high‐risk exposure in the 14 days before illness onset demonstrated lower three‐dose VE during the Omicron predominant period compared with overall VE during this time. These results are consistent with previous studies, including an analysis of data from the US Flu VE Network during the Delta‐predominant period 6,28,29 . Third, the proportion of participants reporting previous laboratory confirmed SARS‐CoV‐2 infection was higher when the Omicron variant predominated than when the Delta variant predominated.…”

“…These results are consistent with previous studies, including an analysis of data from the US Flu VE Network during the Delta-predominant period. 6,28,29 Third, the proportion of participants reporting previous laboratory confirmed SARS-CoV-2 infection was higher when the Omicron variant predominated than when the Delta variant predominated. However, we were unable to evaluate the impact of time since prior infection on VE due to small sample sizes.…”

“…Studies with active enrollment such as the US Flu Vaccine Effectiveness Network (US Flu VE Network) provide access to specimens for research purposes including whole genome sequencing and access to data not available in medical records including risk factors for SARS‐CoV‐2 infection 6 . To assess the impact of a third dose in the context of emerging variants with immune evasion 7 and potential waning immunity, we estimated variant‐specific effectiveness of two versus three mRNA vaccine doses against symptomatic illness in outpatient settings.…”

Effectiveness of 2 and 3 mRNA COVID-19 Vaccines Doses against Omicron and Delta-Related Outpatient Illness among Adults, October 2021 – February 2022

“…Second, participants who reported a high-risk exposure in the 14 days before illness onset demonstrated lower 3-dose VE during the Omicron predominant period compared with overall VE during this time. These results are consistent with previous studies, including an analysis of data from the US Flu VE Network during the Delta-predominant period [6, 29, 30]. Third, the proportion of participants reporting previous laboratory confirmed SARS-CoV-2 infection was higher when the Omicron variant predominated than when the Delta variant predominated.…”

“…Studies with active enrollment such as the US Flu Vaccine Effectiveness Network (US Flu VE Network) provide access to specimens for research purposes including whole genome sequencing and access to data not available in medical records including risk factors for SARS-CoV-2 infection [6]. To assess the impact of a 3 rd dose in the context of emerging variants with immune evasion [7] and potential waning immunity, we estimated variant-specific effectiveness of 2 versus 3 mRNA vaccine doses against symptomatic illness in outpatient settings.…”