A natural experiment during lockdown and on-going care-home COVID-19 outbreaks showed a single dose of vaccine reduced hospitalisation and deaths of care-home residents in North West England

Objective: To examine the effectiveness of one dose of the COVID-19 vaccine on care-home residents. Study design: Natural experiment. Methods: We compared the effectiveness of single doses of Pfizer/BioNTech BNT162b2 (effective at 10 days) and AstraZeneca ChAdOx1 (effective at 14 days) vaccines in vaccinated and control (unvaccinated) homes. Using routine data, all care-homes reporting COVID-19 outbreaks between 11/12/2020 and 12/3/ 2021 in a sub-region of North West England were included. Results: Of 126 care-homes (4042 residents), with outbreaks, 55 (44%, 1686 residents) reported onset dates before vaccination commenced; 38 (30%, 1304 residents) reported onset < 10 (BNT162b2) and < 14 days (ChAdOx1) after vaccine administration; and 33 (26%, 1052 residents) reported onset > 10 (BNT162b2) and > 14 (ChAdOx1) days after vaccination. Eighty-nine (71%) homes used ChAdOx1 vaccine. A single dose of vaccine before the outbreak onset significantly lowered the risk of symptoms (reduced by 48%), positivity (by 65%), hospitalisation (by 68%), and death (by 81%). Some vaccine effectiveness was also noted in care-homes that received one dose of vaccine within 10-14 days of outbreak onset. The number needed to vaccinate to prevent one resident from COVID-19-related hospitalisation was 34, and death was 17. Conclusions: This real-world, natural experiment adds to the evidence of COVID-19 vaccine effectiveness from different studies using varying designs. In the context of lockdown's impact on infection rates and on-going care-home outbreaks, a single dose of either ChAdOx1 or BNT162b2 vaccine had a significant impact on reducing COVID-19 related hospitalisation and death in care-home residents. Natural experiments should be used more in public health. (c) 2024 The Authors. Published by Elsevier Ltd on behalf of The Royal Society for Public Health. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).