Case-Control, Prospective Cohort
Study population and setting
The study analyzed the SARS-CoV-2 infection rate among health care workers in The Sheba Medical Center, the largest hospital in Israel, after vaccination with Pfizer/BioNTech BNT162b2 vaccine. By the end of the study, among 9109 health care workers eligible for the vaccine, 7214 (79%) had received at least one dose and 6037 (66%) had received both doses of the vaccine. Most (91%) of the fully vaccinated workers received their second dose 21 or 22 days following the first dose. SARS-CoV-2 infection was detected through daily required symptom reporting and/or contact tracing followed by PCR lab diagnosis.
Summary of Main Findings
The study found that the vaccine reduced the infection rate (both symptomatic and asymptomatic) from 7.4/10,000 person-days in the unvaccinated group to 5.5/10,000 person-days (30% reduction) and 3/10,000 (75% reduction) on days 1-14 and 15-28 after administration of the first dose of the vaccine, respectively. The symptomatic cases were also reduced by 47% (2.8/10,000 person-days) and 85% (1.2/10,000 person-days) on days 1-14 and 15-28 after administration of the first dose of the vaccine, respectively, compared to the unvaccinated group (5/10,000 person-days).
The study included 9109 health care workers with daily reporting of SARS-CoV-2 symptoms and contact tracing followed by same-day molecular diagnosis of the infection. This allowed for relatively high quality data. The high rate of SARS-CoV-2 infection in Israel at the time in which this study was conducted permitted detection of differences in infection rates among different groups tested in this study.
One of the main limitations of this study is that it included health care workers only. This group has a higher exposure rate relative to the general population. The study also analyzed Pfize/BioNTech vaccine protection for up to 28 days after the first dose. Longer follow up to determine the protection level for one dose relative to two dases of the vaccine. For the effectiveness estimate of the window 15-28 days after vaccination, person-time from vaccinated individuals who had not been infected through 14 days were compared to all person-time contributed by unvaccinated individuals. It is possible that this could induce a selection bias that could overestimate effectiveness during this time window.
This study showed that Pfizer/BioNTech (BNT162b2) vaccine may provide up to 85% protection against symptomatic infection of COVID19 15-28 days after the first dose of the vaccine.