Clinical Effectiveness of SARS-CoV-2 Booster Vaccine Against Omicron Infection in Residents and Staff of Long-term Care Facilities: A Prospective Cohort Study (VIVALDI)

Background Successive SARS-CoV-2 variants have caused severe disease in long-term care facility (LTCF) residents. Primary vaccination provides strong short-term protection, but data are limited on duration of protection following booster vaccines, particularly against the Omicron variant. We investigated effectiveness of booster vaccination against infections, hospitalisations and deaths among LTCF residents and staff in England. Methods We included residents and staff of LTCFs within the VIVALDI study (ISRCTN 14447421) who underwent routine, asymptomatic testing (December 12 2021-March 31 2022). Cox regression was used to estimate relative hazards of SARS-CoV-2 infection, and associated hospitalisation and death at 0-13, 14-48, 49-83 and 84 days after dose 3 of SARS-CoV-2 vaccination compared to 2 doses (after 84+ days), stratified by previous SARS-CoV-2 infection and adjusting for age, sex, LTCF capacity and local SARS-CoV-2 incidence. Results 14175 residents and 19973 staff were included. In residents without prior SARS-CoV-2 infection, infection risk was reduced 0-83 days after first booster, but no protection was apparent after 84 days. Additional protection following booster vaccination waned, but was still present at 84+ days for COVID-associated hospitalisation (aHR: 0.47, 0.24-0.89) and death (aHR: 0.37, 0.21-0.62). Most residents (64.4%) had received primary course of AstraZeneca, but this did not impact on pre- or post-booster risks. Staff showed a similar pattern of waning booster effectiveness against infection, with few hospitalisations and no deaths. Conclusions Booster vaccination provides sustained protection against severe outcomes following infection with the Omicron variant, but no protection against infection from 3 months onwards. Ongoing surveillance for SARS-CoV-2 in LTCFs is crucial.