2083. Waning of Bivalent mRNA Vaccine Effectiveness Against COVID-19-associated Hospitalization Among Immunocompetent Adults Aged ≥65 Years – IVY Network, 20 U.S. States, September 8, 2022-April 1, 2023

BACKGROUND: On September 1, 2022, the Advisory Committee on Immunization Practices recommended a bivalent mRNA COVID-19 booster dose for persons who had completed at least a primary COVID-19 vaccination series ≥2 months earlier. Early data showed high effectiveness of a bivalent booster in preventing COVID-19-associated hospitalization within 45 days of receipt; however, little is known about the durability of this protection. METHODS: Data from the Investigating Respiratory Viruses in the Acutely Ill (IVY) Network were used to conduct a case-control analysis measuring bivalent vaccine effectiveness (VE) against COVID-19–associated hospitalization over time. During September 8, 2022–April 1, 2023, immunocompetent, hospitalized adults aged ≥65 years with COVID-19-like illness were enrolled at 25 hospitals in 20 U.S. states. COVID-19 case-patients tested positive for SARS-CoV-2 by a nucleic acid or antigen test within 10 days of illness onset, while control-patients tested negative for SARS-CoV-2 during the same interval. Multivariable logistic regression was used to measure absolute and relative bivalent VE adjusted for age, sex, race and ethnicity, admission date, and U.S. Health and Human Services region. Unvaccinated patients and patients who received 2–4 doses of monovalent-only mRNA vaccine were used as the reference group for absolute and relative VE, respectively. Bivalent VE was calculated for 7–89 days and 90–179 days from booster dose receipt to illness onset. RESULTS: A total of 2,787 immunocompetent, hospitalized adults aged ≥65 years were enrolled in the IVY Network during the study period (1,236 COVID-19 case-patients and 1,551 control patients). Absolute VE of a bivalent booster dose against COVID-19-associated hospitalization was 58% (95% CI=42%–70%) after 7–89 days and 27% (95% CI= -7% to 50%) after 90–179 days. Relative VE of a bivalent booster dose was 54% (95% CI=41%–64%) after 7–89 days and 19% (95% CI= -8% to 39%) after 90–179 days (Figure). [Figure: see text] CONCLUSION: Bivalent mRNA vaccination provided moderate protection against COVID-19-associated hospitalization within 90 days of receipt among adults aged ≥65 years, with waning protection after 90 days. Additional booster doses could improve protection against COVID-19-associated hospitalization among older adults. DISCLOSURES: Adit A. Ginde, MD, MPH, AbbVie: Grant/Research Support|Faron Pharmaceuticals: Grant/Research Support Ithan Peltan, MD, Asahi Kasei Pharma: Institutional support|Regeneron: Institutional support Emily T. Martin, PhD, MPH, Merck: Grant/Research Support Matthew Exline, MD, Abbott Labs: Honoraria|Regeneron: Grant/Research Support Adam S. Lauring, MD, PhD, Roche: Advisor/Consultant|Sanofi: Advisor/Consultant