The risk of COVID-19 infection gradually increases from 90 days after receiving the second dose of the Pfizer-BioNTech vaccine, according to a study published in The BMJ.
The study by the Research Institute of Leumit Health Services in Israel confirms that vaccine-induced protection wanes with time and suggests a third (booster) dose might be warranted.
Israel was one of the first countries to roll out a large scale COVID-19 vaccination campaign in December 2020, but has seen a resurgence of infections since June 2021.
The findings confirm that the Pfizer vaccine provided excellent protection in the initial weeks after vaccination, but suggest that protection wanes for some individuals with time, the researchers said.
Across the world, large scale COVID-19 vaccination campaigns are helping to control the spread of the virus, but even in countries with high immunisation rates, breakthrough infections can occur, which scientists think is due to a gradual loss of immunity over time.
The researchers examined electronic health records for 80,057 adults with average age 44 years, who received a PCR test at least three weeks after their second dose, and had no evidence of previous COVID-19 infection.
Of these 80,057 participants, 7,973 (9.6 per cent) had a positive test result. These individuals were then matched to negative controls of the same age and ethnic group who were tested in the same week.
These individuals were then matched to negative controls of the same age and ethnic group who were tested in the same week. After taking account of other potentially influential factors, the researchers found a significantly 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 acknowledge that interpretation of their findings is limited by the observational design. They cannot rule out the possibility that other unmeasured factors such as household size, population density, or virus strain may have had an effect.
However, this was a large study of people who all received the same vaccine, and the researchers were able to carry out detailed analysis of the data, suggesting that the results are robust.
They conclude that in individuals who received two doses of the Pfizer vaccine, protection seemed to decrease over time.
The risk of breakthrough infection increased progressively compared with the protection provided during the initial 90 days, according to the researchers.