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ImportancePrevious studies have analyzed effectiveness of booster mRNA Covid-19 vaccination and compared it with 2-dose primary series for both Delta and Omicron variants. Observational studies that estimate effectiveness by comparing outcomes among vaccinated and unvaccinated individuals may suffer from residual confounding and exposure misclassification. ObjectiveTo estimate relative effectiveness of booster vaccination versus the 2-dose primary series with self-controlled study design Design, Setting and ParticipantsWe used the Veterans Health Administration (VHA) Corporate Data Warehouse to identify U.S. Veterans enrolled in care [≥]2 years who received the 2-dose primary mRNA Covid-19 vaccine series and a mRNA Covid-19 booster following expanded recommendation for booster vaccination, and who had a positive SARS-CoV-2 test during the Delta (9/23/2021-11/30/2021) or Omicron (1/1/22-3/1/22) predominant period. Among them, we conducted a self-controlled risk interval (SCRI) analysis to compare odds of SARS-CoV-2 infection during a booster exposure interval versus a control interval. Exposurescontrol interval (days 4-6 post-booster vaccination, presumably prior to gain of booster immunity), and booster exposure interval (days 14-16 post-booster vaccination, presumably following gain of booster immunity) Outcomes and MeasuresPositive PCR or antigen SARS-CoV-2 test. Separately for Delta and Omicron periods, we used conditional logistic regression to calculate odds ratios (OR) of a positive test for the booster versus control interval and calculated relative effectiveness of booster versus 2-dose primary series as (1-OR)*100. The SCRI approach implicitly controlled for time-fixed confounders. ResultsWe found 42 individuals with a positive SARS-CoV-2 test in the control interval and 14 in the booster exposure interval during Delta period, and 137 and 66, respectively, in Omicron period. For the booster versus 2-dose primary series, the odds of infection were 70% (95%CI: 42%, 84%) lower during the Delta period and 56% (95%CI: 38%, 67%) lower during Omicron. Results were similar for ages <65 and [≥]65 years in the Omicron period. In sensitivity analyses among those with prior Covid-19 history, and age stratification, ORs were similar to the main analysis. ConclusionsBooster vaccination was more effective relative to a 2-dose primary series, the relative effectiveness was consistent across age groups and was higher during the Delta predominant period than during the Omicron period.
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BackgroundWe estimated vaccine effectiveness (VE) of mRNA vaccines among US Veterans during periods of Delta and Omicron variant dominance. Patients included in this study were largely 65 years or older (62,834, 55%), male (101,259, 88%), and non-Hispanic white (66,986, 58%). MethodsWe used SARS-CoV-2 laboratory test results to conduct a matched test-negative case-control study to estimate VE of three and two doses of mRNA vaccines against infection (regardless of symptoms), and a matched case-control study to estimate VE against COVID-19-related hospitalization and death. We estimated VE as (1-odds ratio) x 100%. Severity of disease was measured using hospital length of stay (LOS) and admission to an intensive care unit (ICU). ResultsAgainst infection, booster doses had 7-times higher VE - 59% (95% confidence interval [CI], 57 to 61) - than 2-dose VE (7%; 95% CI, 3 to 10) during the Omicron period. For the Delta period, estimated VE against infection was 90% (95% CI, 88 to 92) among boosted vaccinees, 64% higher than VE among 2-dose vaccinees [55% (95% CI, 51 to 58)]. Against hospitalization, booster dose VE was 87% (95% CI, 80 to 91) during Omicron and 95% (95% CI, 91 to 97) during Delta; the 2-dose VE was 44% (95% CI, 26 to 58) during Omicron and 75% (95% CI, 70 to 80) during Delta. Against death, estimated VE with a booster dose was 94% (95% CI, 85 to 98) during Omicron and 96% (95% CI, 88 to 99) during Delta, while the 2-dose VE was 75% (95% CI, 52 to 87) during Omicron and 93% (95% CI, 85 to 97) during Delta. During the Omicron period, average hospital LOS was 4 days shorter [3 days (95%CI, 3 to 4 days)] than during the Delta period. ConclusionsA mRNA vaccine booster is more effective against infection, hospitalization, and death than 2-dose vaccination among an older male population with comorbidities.
