Effectiveness of Bivalent Boosters Over Nine and Half Months

Author(s): Sai Paritala, Yangjianchen Xu, Yi Du, Matthew Donahue, Patrick Maloney, Dan-Yu Lin

On September 1, 2022, Moderna and Pfizer–BioNTech bivalent vaccines replaced existing monovalent vaccines as booster doses against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) for persons aged 12 and older in the United States. We assessed the effectiveness of these bivalent boosters against Omicron infection and severe outcomes (COVID-19 hospitalization and death) over a 9.5-month period using line-level data from the state of Nebraska. We found that the relative effectiveness of bivalent boosters, compared with one fewer vaccine dose, against Omicron infection and subsequent death was 39.0% (95% confidence interval [CI], 35.7 to 42.2) and 70.0% (95% CI, 38.8 to 85.3), respectively, at four weeks post administration and gradually waned afterward. Vaccine effectiveness was broadly similar against different Omicron subvariants.