Relative Vaccine Effectiveness of a SARS-CoV-2 mRNA Vaccine Booster Dose Against the Omicron Variant.

ABSTRACT

BACKGROUND: The current SARS-CoV-2 vaccines may be less effective against the Omicron variant. With recent resurgence of SARS-CoV-2 cases, the role of booster doses of the vaccine needs to be highlighted. METHODS: Using a retrospective cohort study design emulating a target trial, we determined the relative effectiveness of a homologous booster dose of a SARS-CoV-2 mRNA vaccine compared with primary series alone in preventing infection, hospitalization, and intensive care unit (ICU) admission, and death in the Department of Veterans Affairs healthcare system in the US. Among infection-free survivors who received two doses of an mRNA vaccine prior to April 30, 2021, we identified those who received a booster between September 22 and December 25, 2021 and 11 matched individuals who did not receive a booster. RESULTS: Among 2,384,272 previously uninfected persons with two doses of an mRNA vaccine by April 30, 2021, we identified 462,950 booster recipients between September 22 and December 25, 2021 who were matched 11 with non-booster recipients. RVE (95% CI) was 19% (17-22%) for confirmed infection, 52% (46-57%) for hospitalization, and 83% (65-92%) for ICU admission or death. Recipients of the mRNA-1273 vaccine had a lower cumulative incidence of infections and hospitalizations compared with BNT-162b2 vaccine (log-rank p-value <0.001 for both comparisons). CONCLUSION: While the RVE of SARS-CoV-2 mRNA booster vaccine dose in preventing infection against the Omicron variant is low, the RVE is substantial in preventing hospitalization and high in preventing the most severe/critical disease.