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To the Editor: The surge of coronavirus disease 2019 (Covid-19) around the world and the need for urgent and intensive medical care have weighed heavily on health care systems and hospitals. Health care workers are at high risk of exposure to Covid-19, both in the community and in the workplace when providing care to patients.1 Covid-19–associated isolation and quarantine of health care workers place additional burdens on health care services. Since the introduction of vaccines, prioritizing vaccination of health care workers has been advocated, and data on vaccine effectiveness among health care workers in real-world settings is beginning to emerge.

–associated isolation and quarantine of health care workers place additional burdens on health care services. Since the introduction of vaccines, prioritizing vaccination of health care workers has been advocated, and data on vaccine effectiveness among health care workers in real-world settings is beginning to emerge. We examined vaccine effectiveness among health care workers at the Hadassah Hebrew University Medical Center (HHUMC), a two-campus medical center in Jerusalem that employs 6680 people. Jerusalem has one of the highest incidence rates of Covid-19 in Israel.2 In addition to its regular activities, HHUMC is currently treating patients with Covid-19 in eight dedicated wards. To create a safe hospital environment, HHUMC established a proactive, periodic screening program for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) for all personnel.3 From the beginning of the epidemic through January 31, 2021, of the 6680 health care workers, 689 (10.3%) were infected, mostly due to exposure to Covid-19 in the community; the trends in incidence among health care workers were similar to that in the Jerusalem population (Fig. S1 in the Supplementary Appendix, available with the full text of this letter at NEJM.org). Vaccination with two doses of the Pfizer–BioNtech vaccine, given 21 days apart,4 began on December 20, 2020. Within 8 weeks, 5297 of 6252 (84.7%) health care workers who had not been previously infected by December 20 were vaccinated. Most of the health care workers (98.9%) who had received the first dose of vaccine and were not infected by day 21 received the second dose. We collected data regarding the vaccine status of health care workers and the infections that occurred among them. Among the vaccinated workers, the weekly incidence of Covid-19 since the first dose declined notably after the second week; the incidence of infection continued to decrease dramatically and then remained low after the fourth week (Table 1 and Fig. S2). Since September 2020, the probability of being free from Covid-19 had steadily decreased until the commencement of the two-dose vaccinations, after which infections among vaccinated health care workers occurred far less often (Fig. S3). Of note, the numerator used to calculate the incidence of Covid-19 was the number of patients with infection that was detected on either mandatory or voluntary testing; systematic testing of all vaccinated health care workers was not performed.

ons among vaccinated health care workers occurred far less often (Fig. S3). Of note, the numerator used to calculate the incidence of Covid-19 was the number of patients with infection that was detected on either mandatory or voluntary testing; systematic testing of all vaccinated health care workers was not performed. Therefore, we may have missed some positive cases. In our study that was conducted in an active hospital setting in a community with a high incidence of Covid-19, vaccination of health care workers with the BNT162b2 vaccine resulted in a major reduction of new cases of Covid-19 among those who received two doses of the vaccine, even when a surge of the B.1.1.7 variant was noted in up to 80% of cases.5 These findings suggest that widespread and effective vaccination among health care workers provides a safe environment, even in the presence of a high rate of SARS-CoV-2 infection in the community.