Vaccine effectiveness of the first dose of ChAdOx1 nCoV-19 and BNT162b2 against SARS-CoV-2 infection in residents of Long-Term Care Facilities (VIVALDI study)

Shrotri, M.; Krutikov, M.; Palmer, T.; Giddings, R.; Azmi, B.; Subbarao, S.; Fuller, C.; Irwin-Singer, A.; Davies, D.; Tut, G.; Lopez Bernal, J.; Moss, P.; Hayward, A.; Copas, A.; Shallcross, L.

Background The effectiveness of SARS-CoV-2 vaccines in frail older adults living in Long-Term Care Facilities (LTCFs) is uncertain. We estimated protective effects of the first dose of ChAdOx1 and BNT162b2 vaccines against infection in this population. Methods Cohort study comparing vaccinated and unvaccinated LTCF residents in England, undergoing routine asymptomatic testing (8 December 2020 - 15 March 2021). We estimated the relative hazard of PCR-positive infection using Cox proportional hazards regression, adjusting for age, sex, prior infection, local SARS-CoV-2 incidence, LTCF bed capacity, and clustering by LTCF. Results Of 10,412 residents (median age 86 years) from 310 LTCFs, 9,160 were vaccinated with either ChAdOx1 (6,138; 67%) or BNT162b2 (3,022; 33%) vaccines. A total of 670,628 person days and 1,335 PCR-positive infections were included. Adjusted hazard ratios (aHRs) for PCR-positive infection relative to unvaccinated residents declined from 28 days following the first vaccine dose to 0.44 (0.24, 0.81) at 28-34 days and 0.38 (0.19, 0.77) at 35-48 days. Similar effect sizes were seen for ChAdOx1 (aHR 0.32 [0.15-0.66] and BNT162b2 (aHR 0.35 [0.17, 0.71]) vaccines at 35-48 days. Mean PCR cycle threshold values were higher, implying lower infectivity, for infections 28 or more days post-vaccination compared with those prior to vaccination (31.3 vs 26.6, p<0.001). Interpretation The first dose of BNT162b2 and ChAdOx1 vaccines was associated with substantially reduced SARS-CoV-2 infection risk in LTCF residents from 4 weeks to at least 7 weeks. Funding UK Government Department of Health and Social Care.