Among adults who received 2 doses of the Pfizer-BioNTech (BNT162b2) mRNA COVID-19 vaccine, the risk for SARS-CoV-2 infection increased gradually after 90 days, according to results of a study published in BMJ.

Researchers performed a test negative design study using electronic health records from a large state mandated healthcare organization in Israel. Participants were aged 18 years and older, had received a reverse transcription polymerase chain reaction (RT-PCR) test between 15 May 2021 and 17 September 2021 at least 3 weeks after their second COVID-19 vaccine dose, had not received a third vaccine dose, and had no history of SARS-CoV-2 infection. Participants who tested positive for SARS-CoV-2 via RT-PCR were matched against healthy controls on the basis of week of RT-PCR testing, age, and demographic group.

Among a total of 83,057 participants who received an RT-PCR test during the study period, 9.6% received a positive result. In addition, the researched noted that time elapsed since COVID-19 vaccination was significantly increased among participants who tested positive for SARS-CoV-2 infection (P <.001). The adjusted odds ratio (aOR) for SARS-CoV-2 infection following vaccination was significantly increased at day 90 or greater compared with the reference of less than 90 days. Further analysis showed that the aOR for SARS-CoV-2 infection following vaccination was 2.37 between days 90 and 119 (95% CI, 1.67-3.36), 2.66 between days 120 and 149 (95% CI, 2.07-3.84), 2.82 between days 150 and 179 (95% CI, 2.07-3.84), and 2.82 at day 180 or greater (95% CI, 2.07-3.85; P <.001 for each 30-day interval).


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The study was limited by potential unmeasured confounders due to its observational design. The researched noted that the study did not assess the severity of SARS-CoV-2 among participants who developed breakthrough infections following vaccination.

According to the researchers, “interpretation of the findings of [this] study is limited by the observational design, but the results suggest that consideration of a third [COVID-19] vaccine dose might be warranted.”

Reference

Israel A, Merzon E, Schäffer AA, et al. Elapsed time since BNT162b2 vaccine and risk of SARS-CoV-2 infection: test negative design study. BMJ. 2021;375:e067873. doi: 10.1136/bmj-2021-067873