Effectiveness of a SARS-CoV-2 mRNA vaccine booster dose for prevention of infection, hospitalization or death in two nation-wide nursing home systems

Background A SARS-CoV-2 vaccine booster dose has been recommended for all nursing home residents. However, we lack effectiveness data on boosters preventing infection, death and hospitalization in this frail population. Methods We emulated nested target trials in two large nursing home systems in parallel to evaluate the effectiveness of a SARS-CoV-2 mRNA vaccine booster at preventing infection, hospitalization, or death. Residents who completed a 2-dose series of the mRNA vaccine and were eligible for a booster were included in from September 22, 2021 to November 5, 2021. Outcomes were measured through December 18, 2021, including test-confirmed SARS-CoV-2 infection, hospitalization, or death. The vaccine effectiveness at day 42 was estimated with a Kaplan-Meier estimator, both unadjusted and weighted with the inverse probability of treatment. Results The two NH systems were large and multi-state, System 1 included 200 NH (8,538 control and 5,721 boosted residents) and System 2 included 127 NHs (4,100 control and 2,291 boosted residents). Booster vaccination reduced infections by 50.4% (95% Confidence Interval [CI]: 29.4%, 64.7%) SARS-CoV-2 infections in System 1 and 58.2% (32.3%, 77.8%) in System 2. Boosted residents in System 1 also had a 97.3% (86.9%, 100.0%) reduction in SARS-CoV-2 associated death, but too few events for comparison in System 2. Conclusions During a Delta predominant period, SARS-CoV-2 booster vaccination significantly reduced infection in two U.S. nursing home systems. In the larger System 1 a 97% reduction in SARS-CoV-2 related death was also observed. These findings strongly support administration of vaccine boosters to nursing home residents.