The booster dose of the Pfizer-BioNTech mRNA vaccine reduced the chances of testing positive for SARS-CoV-2 by 86%, according to preliminary results from a short-term Israeli study reported in JAMA Internal Medicine on Tuesday.¹

The retrospective case-control study included 306,710 patients aged 40 years and older who were a part of an Israeli healthcare maintenance organization covering 2.5 million members. They were administered either 2 or 3 doses of the Pfizer-BioNTech (BNT162b2) mRNA vaccine between March 2020 and October 2021. None of them were positive for SARS-CoV-2 before the start of the follow up period. A test-negative design and a matched case-control design were used for the study. Data was analyzed mainly from August 1 to October 4, 2021. Out of a total of 500,232 RT PCR tests carried out during the entire duration of the study, 227,380 persons were given two doses, while 272,852 received three doses. The number of patients testing positive was 14,989 (6.6%) and 4941 (1.8%), respectively in the two groups.

Results showed that the chances of infection reduced considerably within a few weeks for those participants who got the third dose of the vaccine vis à vis those who had only completed their primary vaccination. The odds of testing positive for SARS-CoV-2 reduced by 86% after receiving the booster dose. This reduction was in addition to the reduction in the risk accorded by the first two doses.

The effectiveness of the booster was enhanced with time. While the reduction in risk of infection was 12% in the first week after booster, it increased to 58% in the second week and rose to 85% after two weeks. The authors write, “The positive percentage was highest among those who had not received a booster and among those who had received a booster in the previous 7 days, and was lowest among individuals who had received the booster more than 2 weeks prior.”

This study has shown the third dose of the Pfizer vaccine considerably reduced the chances of getting infected with SARS-CoV-2 and hospitalization. The booster dose enhanced the waning vaccine-induced immunity as has been shown earlier in many studies and provided extra protection.

Reference

1. Patalon T, et al. Odds of testing positive for SARS-CoV-2 following receipt of 3 vs 2 doses of the BNT162b2 mRNA vaccine. JAMA Intern Med. 2021 Nov 30. doi: 10.1001/jamainternmed.2021.7382.