Effectiveness of mRNA COVID-19 Vaccines against Omicron among Veterans

Background: Effectiveness of mRNA vaccinations against Omicron among US veterans is unknown. Objectives: To estimate mRNA vaccine effectiveness (VE) as measured by risk of COVID infection. Design, Setting, and Participants: This matched test negative case control study was conducted using COVID test results at Veterans Health Administration sites from December 1, 2021, to December 31, 2021. VE against COVID infection and COVID related hospitalization and death were estimated using electronic health records from veterans who routinely sought care at a VHA facility and had a test result positive for COVID (cases) or negative for COVID (controls). Data were analyzed from December 1, 2021 to January 14, 2022. Exposures: Vaccination status, defined as number of mRNA vaccine doses (1, 2, 3 or more) as the primary explanatory variable, at time of test. Main Outcomes and Measures: The main outcome of interest was a positive result for COVID on a polymerase chain reaction or antigen test. Secondary outcomes included COVID related hospitalization and death, defined by discharge data and proximity of event to positive test result. VE was estimated from odds ratios for COVID infection with 95% CIs. Results: VE for those who have received a mRNA vaccine booster is 77% (95%, 75 to 79) against Delta while 62% (95%, 59 to 65) against Omicron infection, 91% (95%, 85 to 94) against COVID related hospitalization with better VE against Omicron, and 96% (95%, 91 to 98) against COVID related death with no discernable difference between Omicron and Delta. However, without a booster, VE against COVID related death decreases to 76% (95%, 62 to 85). Conclusions: Although Omicron is highly contagious and more likely to result in breakthrough infections against the current mRNA vaccines, a mRNA vaccine booster is still effective against severe COVID related outcomes.