Findings of a large observational study from Israel validate the protective effect of the third dose of the Pfizer-BioNTech vaccine against severe COVID-19-related outcomes, compared with receiving only two doses at least 5 months ago.

The study evaluated persons who had received a third dose of the Pfizer-BioNTech vaccine between July 30, 2020 and September 23, 2021. The control group included individuals who had not taken the third dose. Data was sourced from one of the largest health-care organizations providing mandatory health-care coverage in Israel. Persons who had taken the second dose of the vaccine at least five months before enrolment in the study and were eligible for the third dose, no past history of Covid-19 and no visit to the healthcare system three days before the recruitment date.

The study participants were grouped into two: Third dose group, where the eligible persons were administered the third dose at study entry and the control group, who did not receive the third dose at any time during follow-up. Each group included 728,321 individuals with a median age of 52 years. Women comprised more than half of the participants. The effectiveness of the third dose was assessed ≥7 days of administration of the vaccine.

Compared with the two doses of the vaccine received at least 5 months ago, the effectiveness of the third dose was found to be 93% (231 events for two doses vs 29 events for three doses) against admission to hospital, 92% (157 vs 17 events) against severe disease and 81% (44 vs 7 events) against Covid-19-related mortality. The effectiveness of the third dose against documented SARS-CoV-2 infection was 88% (6131 events for two doses vs 1135 events for three doses) and 91% (3345 vs 514 events) against symptomatic infection.

These findings show that compared to the patients who had received two doses only at least 5 months before their recruitment in this study, the third dose recipients had fewer adverse Covid-19-related outcomes in terms of hospitalization for severe disease and/or mortality.

Reference

1. Barda N, et al. Effectiveness of a third dose of the BNT162b2 mRNA COVID-19 vaccine for preventing severe outcomes in Israel: an observational study. Lancet. October 29; https://doi.org/10.1016/S0140-6736(21)02249-2.