In a new study from the Research Institute of Leumit Health Services in Israel, researchers found a gradual increase in the risk of COVID-19 infection from 90 days after receiving a second dose of the Pfizer-BioNTech vaccine.
The study was carried out by. Israel was one of the first countries to roll out a large scale COVID-19 vaccination campaign in December 2020, but which has seen a resurgence of infections since June 2021.
The findings confirm that the Pfizer-BioNTech vaccine provided excellent protection in the initial weeks after vaccination, but suggest that protection wanes for some individuals with time.
Across the world, large scale COVID-19 vaccination campaigns are helping to control the spread of the virus, but even in countries with high vaccination rates, breakthrough infections can occur, which scientists think is due to a gradual loss of immunity over time.
In the study, the team examined electronic health records for 80,057 adults (average age 44 years) who received a PCR test at least three weeks after their second injection, and had no evidence of previous COVID-19 infection.
Of these 80,057 participants, 7,973 (9.6%) had a positive test result. The rate of positive results increased with time elapsed since a second dose.
For example, across all age groups, 1.3% of participants tested positive 21-89 days after a second dose, but this increased to 2.4% after 90-119 days; 4.6% after 120-149 days; 10.3% after 150-179 days; and 15.5% after 180 days or more.
And after taking account of other potentially influential factors, the researchers found a strongly increased risk of infection with time elapsed since a second dose.
Compared with the initial 90 days after a second dose, the risk of infection across all age groups was 2.37-fold higher after 90-119 days; 2.66-fold higher after 120-149 days; 2.82-fold higher after 150-179 days; and 2.82-fold higher after 180 days or more.
The researchers conclude that in individuals who received two doses of the Pfizer-BioNTech vaccine, protection seemed to decrease over time, and the risk of breakthrough infection increased progressively compared with the protection provided during the initial 90 days.
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The study is published in The BMJ.
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