Recombinant Influenza Vaccine and Influenza Outbreaks in US Nursing Homes: Results from a Pragmatic Cluster-Randomized Clinical Trial

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Abstract Background Influenza outbreaks in U.S. nursing homes contribute substantially to morbidity, mortality, and operational strain. Recombinant influenza vaccine may reduce transmission in long-term care settings, but high-quality evidence to support this effect is limited. To compare the risk of influenza outbreaks (confirmed outbreak: 1+ laboratory-confirmed cases; suspected outbreak 2+ influenza-like illness cases) among nursing homes randomized to recombinant quadrivalent influenza vaccine (RIV4) versus standard-dose egg-based inactivated quadrivalent influenza vaccine (IIV4). Methods This study was a pragmatic, cluster-randomized clinical trial conducted across two influenza seasons (n=965 facilities 2019–2020 and n=1,023 in 2020–2021) in Medicare-certified U.S. nursing homes. Nursing homes with ≥50 long-stay residents and high proportions of adults aged ≥65 years were eligible and assigned to RIV4 or IIV4 for resident and staff vaccination. The outcome was facility-reported influenza outbreaks, assessed by monthly logs. Results During the 2019–2020 season, RIV4-assigned facilities experienced fewer suspected (148 [30.6%] vs 178 [37.0%]) and laboratory-confirmed (177 [36.6%] vs 214 [44.5%]) influenza outbreaks than IIV4 facilities. The adjusted relative risk (RR) for confirmed outbreaks was 0.82 (95% Confidence Interval, 0.71–0.96). In 2020–2021, confirmed outbreaks were rare (16 vs 15), and no significant differences were observed. COVID-19 outbreak risk did not differ between groups in either season. Conclusions RIV4 reduced influenza outbreak occurrence in U.S. nursing homes during a season with moderate influenza activity in older adults. No differences were observed during the low-circulation 2020–2021 season. These findings suggest that recombinant influenza vaccines may provide clinically meaningful facility-level benefits in long-term care settings during seasons with substantial influenza circulation.