426. Effectiveness of Updated (2023-2024) Monovalent COVID-19 Vaccines – VISION Network, September 2023-March 2024

Abstract Background In September 2023, the Centers for Disease Control and Prevention’s Advisory Committee on Immunization Practices (ACIP) recommended updated 2023-2024 (monovalent XBB.1.5) COVID-19 vaccines for use in persons ≥6 months for prevention of COVID-19, including severe disease. Understanding how well updated COVID-19 vaccines work in the context of high population immunity due to prior infection, vaccination, or both, is important for future vaccine policy decisions, as well as informing patient/provider discussions, and increasing vaccine confidence. Effectiveness of updated (2023-2024) COVID-19 vaccination against laboratory-confirmed COVID-19-associated emergency department/urgent care encounters and hospitalization among immunocompetent adults aged ≥18 years — VISION Network, September 2023–March 2024 Methods VISION, an electronic health record (EHR)-based network including emergency departments/urgent care clinics (ED/UCs) and hospitals including 6 health systems, uses clinician-ordered testing data to estimate vaccine effectiveness (VE) for respiratory viruses. COVID-19 VE was estimated using the test-negative design, comparing the odds of vaccination with a single updated 2023-2024 COVID-19 vaccine dose between patients who tested positive versus those who tested negative for SARS-CoV-2 by molecular assay, adjusting for potential confounders. Effectiveness of updated 2023–2024 (monovalent XBB.1.5) COVID-19 vaccination against laboratory-confirmed COVID-19–associated hospitalization among immunocompromised adults aged ≥18 years — VISION network, September 2023–March 2024 Results A total of 198,345 ED/UC encounters and 59,777 hospitalizations between September 2023-March 2024 among immunocompetent adults ≥18 years with symptoms of COVID-19 were included. VE against COVID-19-associated ED/UC encounters comparing receipt of an updated dose to no receipt of an updated dose was 50% (95% CI: 46-53%) in the 7-59 days and 0 (95% CI: -14 to 12) in the 120-179 days after receipt of an updated dose (Table 1). VE against COVID-19-associated hospitalization was 50% (95% CI: 44-56%) in the 7-59 days and 23 (95% CI: 2-39) in the 120-179 days after receipt of an updated dose (Table 1). A total of 17,417 hospitalizations among immunocompromised adults ≥18 years with symptoms of COVID-19 were included, with a VE of 38% (95% CI: 23-50%) in the 7-59 days and 14 (95% CI: -33 to 44) in the 120-179 days after receipt of an updated dose after an updated dose (Table 2). Conclusion Receipt of an updated COVID-19 vaccine dose provided protection against COVID-19-associated ED/UC encounters and hospitalizations among immunocompetent and immunocompromised adults, although waning was evident. Disclosures Nicola P. Klein, MD, PhD, GlaxoSmithKline: Grant/Research Support|Merck: Grant/Research Support|Pfizer: Grant/Research Support|Sanofi Pasteur: Grant/Research Support|Seqirus: Grant/Research Support Toan Ong, PhD, Patent Title: Systems and Methods For Record Linkage: PCT/US2018/047961 Patent Title: Systems and Methods For Record Linkage|PCORI: Travel support to attend the PCORI Annual meeting in Washington DC, 2023|Regenstrief Institute: Advisor/Consultant|Regenstrief Institute: Travel support to attend the OHIE 23 meeting in Malawi. Brian E. Dixon, PhD, Elsevier: Honoraria