Background and Objectives:
Given the mass vaccination campaigns to control COVID-19 pandemic, it is important to measure the effect of vaccination against COVID-19, in real conditions. The Portuguese Navy, a population at high risk of exposure due to its functions of defence and support to the population in the scope of the pandemic were eligible to early vaccination in December 2020. Using data from the Portuguese Navy electronic health registries, this study intended to estimate COVID-19 vaccine effectiveness in the Portuguese Navy, against laboratory-confirmed SARS-CoV-2 infection, overall and by time since complete primary vaccination.

Methods:
We stablished a retrospective and prospective cohort that took place for 20months (December 2020-June 2022). The study population was composed of active-duty and reserve militaries of the Portuguese Navy, all eligible for COVID-19 vaccination that consented to participate in this study. Invitation was via e-mail and consent was obtained to link data from health registries and vaccination. Complete scheme vaccine effectiveness (VE) was estimated as 100%*(1-HR) where HR is the confounding-adjusted hazard ratio from time-dependent Cox regression, adjusted for age, sex, and presence of chronic conditions.

Results:
A total of 860 Portuguese Navy militaries accepted to participate in the study. Complete primary scheme overall VE against laboratory-confirmed SARS-CoV-2 infection was 54.0% (-5.7 to 79.9) after 14 days of last dose. This estimate was 73% (16.5 to 90.1), 14 to 97 days after the conclusion of the primary vaccination scheme and a waning effect was observed for 98 to 181 days and 182+ days after primary vaccination, 47.6% (-43.5 to 80.9) and 37.8% (-60.4 to 75.9), respectively.

Conclusion:
Regarding overall VE estimates against laboratory confirmed SARS-CoV-2, according to time since complete vaccination, we observed a waning effect, similar to the general population. These results should be interpreted with caution given the high uncertainty of the estimates.