Effectiveness of Monovalent mRNA Vaccines Against COVID-19–Associated Hospitalization Among Immunocompetent Adults During BA.1/BA.2 and BA.4/BA.5 Predominant Periods of SARS-CoV-2 Omicron Variant in the United States — IVY Network, 18 States, December 26, 2021–August 31, 2022

The SARS-CoV-2 Omicron variant (B.1.1.529or BA.1) became predominant in the United States by late December 2021 (1).BA.1 has since been replaced by emerging lineages BA.2 (including BA.2.12.1) in March 2022, followed by BA.4 and BA.5, which have accounted for a majority of SARS-CoV-2 infections since late June 2022 (1).Data on the effectiveness of monovalent mRNA COVID-19 vaccines against BA.4/BA.5associatedhospitalizations are limited, and their interpretation is complicated by waning of vaccine-induced immunity (2-5).Further, infections with earlier Omicron lineages, including BA.1 and BA.2, reduce vaccine effectiveness (VE) estimates because certain persons in the referent unvaccinated group have protection from infection-induced immunity.The IVY Network † assessed effectiveness of 2, 3, and 4 doses of monovalent mRNA vaccines compared with no vaccination against COVID-19associated hospitalization among immunocompetent adults aged ≥18 years during December 26, 2021-August 31, 2022.During the BA.1/BA.2period, VE 14-150 days after a second dose was 63% and decreased to 34% after 150 days.Similarly, * These authors contributed equally to this report.