Effectiveness of the first dose of BNT162b2 vaccine to preventing COVID-19 in healthcare personnel

Background: A first protective dose of vaccine may allow delaying the second dose in a context of low supply. The objective is to assess the effectiveness of a single dose of vaccine against SARS-CoV-2 (BNT162b2) after twelve days of its administration in healthcare personnel (HCP) of a Health Department. Methods: A case-control study was made. HCP with suspected COVID-19 and HCP close contacts of COVID-19 cases were included between January 27 and February 7, 2021. They were PCR tested for SARS-CoV-2; those with positive PCR were considered cases and those with negative PCR were considered controls. The crude (VE) and adjusted (VEa) vaccine effectiveness to prevent COVID-19 cases and their 95% confidence interval were calculated using the formula VE = (1-Odds ratio) x 100. Results: 268 HCP were included, of which 70 (26.1%) were considered cases and 198 (73.9%) controls. The frequency of vaccine exposure in cases was 55.7% vs. 69.7% in controls (p=0.035). The VEa of the first vaccine dose was 52.6% (95%CI: 1.1-77.3). The VEa in the subgroup of HCP studied for suspected disease was 74.6% (CI95%: 38.4-89.5). Conclusions: One dose of BNT162b2 vaccine against SARS-CoV-2 offers early protection after twelve days of administration. These data could be considered to adapt strategies and consider postponing the second dose in situations of limited vaccine supply in order to achieve the maximum number of people covered with a first dose.