There is a gradual increase in the risk of COVID-19 infection from 90 days after receiving a second dose of the Pfizer-BioNTech vaccine, according to a new Israeli study.
Writing in today’s BMJ, a team at the Research Institute of Leumit Health Services say their findings suggest that a third “booster” dose could be warranted.
They say while the Pfizer-BioNTech vaccine provided excellent protection in the initial weeks after vaccination, for some individuals it wanes with time.
They examined electronic health records for 80,057 adults, whose average was 44 years, who received a PCR test at least three weeks after their second injection and had no evidence of previous COVID-19 infection.
Out of the total, 7,973 (9.6%) had a positive test result. These were then matched to negative controls of the same age and ethnic group who were tested in the same week.
The researchers found the rate of positive results increasing as time elapsed following the second dose.
Across all age groups, 1.3% of participants tested positive 21-89 days after a second dose and this increased to 2.4% after 90-119 days; 4.6% tested positive after 120-149 days; 10.3% after 150-179 days; and 15.5% after 180 days or more.
After taking account of other potentially influential factors, the research team found a significantly increased risk of infection with time elapsed since a second dose.
Compared with the first 90 days after a second dose, the risk of infection across all age groups was 2.37-fold higher after 90-119 days. After 120-149 days it was 2.66-fold higher, and 2.82-fold higher after 150-179 days. After 180 days, it was 2.82-fold higher.
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 25 November 2021
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