MONDAY, Nov. 29, 2021 (HealthDay News) — The risk of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection increases at time intervals >90 days since the second dose of Pfizer-BioNTech BNT162b2 mRNA vaccine, according to a study published online Nov. 25 in The BMJ.

Ariel Israel, M.D., Ph.D., from Leumit Health Services in Tel Aviv, Israel, and colleagues examined the association between time elapsed since the second injection of BNT162b2 mRNA vaccine and risk of COVID-19 infection in a study involving adults aged ≥18 years who received a reverse transcription polymerase chain reaction (RT-PCR) test between May 15 and Sept. 17, 2021, at least three weeks after their second vaccine dose.

During the study period, 83,057 adults received a RT-PCR test and 9.6 percent had a positive result for SARS-CoV-2. The researchers found that individuals who tested positive had significantly longer time elapsed since vaccine injection. Compared with the reference of <90 days, the adjusted odds ratios for infection at time intervals >90 days since vaccination were significantly elevated: 2.37, 2.66, 2.82, and 2.82 for 90 to 119 days, 120 to 149 days, 150 to 179 days, and ≥180 days, respectively.

“In this retrospective large cohort study, performed in individuals who received two doses of the BNT162b2 mRNA vaccine, protection seemed to decrease over time, and the risk of breakthrough infection by SARS-CoV-2 increased progressively compared with the protection provided during the initial 90 days,” the authors write. “The results suggest that consideration of a third vaccine dose might be warranted.”

Several authors are employees of Leumit Health Services, which partially funded the study.

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