Covid-19 vaccine effectiveness against general SARS-CoV-2 infection from the omicron variant: A retrospective cohort study

ObjectiveTo estimate the effectiveness of 2-dose and 3-dose mRNA vaccination (BNT162b2 and mRNA-1273) against general SARS-CoV-2 infection (asymptomatic or symptomatic) caused by the omicron variant. DesignPropensity-score matched retrospective Cohort Study. SettingLarge public university undergoing weekly Covid-19 testing in South Carolina, USA. ParticipantsPopulation consists of 24,145 university students and employees undergoing weekly Covid-19 testing between January 3rd and January 31st, 2022. The analytic sample was constructed via propensity score matching on vaccination status: Unvaccinated, completion of 2-dose mRNA series within previous 5 months, and receipt of mRNA booster dose within previous 5 months. The resulting analytic sample consists of 1,944 university students and 658 university employees. InterventionVaccination with a two dose or 3 dose regimen of the BNT162b2 or mRNA-1273 vaccine. ResultsBooster protection against any SARS-CoV-2 infection was 66.4% among employees (95% CI: 46.1-79.0%; P<.001) and 45.4% among students (95% CI: 30.0-57.4%; P<.001). Compared to the 2-dose mRNA series, estimated increase in protection from the booster dose was 40.8% among employees (P=.024) and 37.7% among students (P=.001). We did not have enough evidence to conclude a statistically significant protective effect of the 2-dose mRNA vaccination series, nor did we have enough evidence to conclude that protection waned in the 5-month period after receipt of the 2nd or 3rd mRNA dose. Furthermore, we did not find evidence that protection varied by manufacturer. ConclusionsCovid-19 mRNA booster doses offer moderate protection against any SARS-CoV-2 infection caused by the omicron variant and provide a substantial increase in protection relative to the 2-dose mRNA vaccination series.