Continuous assessment of the effectiveness of approved COVID-19 vaccines is crucial to gain an insight into the longer-term impact on health outcomes, and eventually boosting public confidence. For this reason, we conducted a multicenter, retrospective cohort study using data on infection and vaccination rates among employees of three Prague hospitals in the period between 27 December 2020 and 31 August 2021. The post-vaccination and post-infection protectiveness were assessed in a total of 11,443 hospital workers who were followed up for more than 14 days either after their Comirnaty vaccination or study enrolment, depending on their previous SARS-CoV-2 infection. The effectiveness of full vaccination against any SARS-CoV-2 infection achieved 88.3% (83.2-91.8%) over the eight months of follow-up, a figure not much different from the 92.5% (76.5-97.6%) level of protection built by a previous infection. Despite this, the post-vaccination level of protection declined to about 65% between June and August. No case of breakthrough infection was registered among hospital workers having received one or two vaccine doses more than three months after previous infection. The eight-month effectiveness of the Comirnaty vaccine exhibited a declining trend requiring a new booster dose. The need for vaccination in the previously infected employees was not demonstrated conclusively in this study.

1st Faculty of Medicine Charles University and Military University Hospital 121 08 Prague Czech Republic

3rd Faculty of Medicine Charles University 100 00 Prague Czech Republic

Bulovka University Hospital 180 00 Prague Czech Republic

Institute for Clinical and Experimental Medicine 140 21 Prague Czech Republic

Královské Vinohrady University Hospital 100 34 Prague Czech Republic

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