Cohort study of Covid-19 vaccine effectiveness among healthcare workers in Finland, December 2020-October 2021

Recently, Covid-19 vaccine effectiveness has decreased especially against mild disease due to emergence of the Delta variant and waning protection. In this register-based study among healthcare workers in Finland, the vaccine effectiveness of two-dose mRNA vaccine series against SARS-CoV-2 infection decreased from 82% (95% CI 79-85%) 14-90 days after vaccination to 53% (43-62%) after 6 months. Similar trend was observed for other series. Waning was not observed against Covid-19 hospitalization. These results facilitate decision-making of booster doses for healthcare workers (c) 2021 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Automated summary

Recently, the effectiveness of Covid-19 vaccines, especially against mild disease, has decreased due to the emergence of the Delta variant and waning protection. A register-based study in Finland found that the vaccine effectiveness of two-dose mRNA vaccine series against SARS-CoV-2 infection decreased from 82% to 53% after 6 months. Similar trends were observed for other vaccine series. Waning protection was not observed against Covid-19 hospitalization. These results inform decision-making regarding booster doses for healthcare workers.