A study published by the BMJ today shows an increase in the risk of developing covid-19 90 days after receiving another dose of the Pfizer BioNTech vaccine.
The study was conducted by the Research Institute of Leumit Health Services in Israel. Israel was one of the first countries to introduce large-scale covid-19 vaccination campaign in the month of December 2020, however, it has seen a return of infections since June 2021.
The results confirm that Pfizer-BioNTech vaccine offered excellent protection in the initial weeks after vaccination, but suggests that protection diminishes for certain patients over time.
Large-scale vaccination campaigns for covid-19 across the globe are helping stop the spread of the virus. However, breakthrough infections can still occur in countries that have high vaccination rates. Scientists believe this is due to a gradual loss in immunity over time.
The interval between vaccinations and the possibility of infection could provide valuable information about the need for a 3rd injection.
Researchers looked through the electronic health records of 80,057 adults (average 44 years old) who had received a PCR test within three weeks of their second injection. There was no evidence to suggest that they had ever been infected by covid-19.
Of these 80,057 participants, 7,973 (9.6 percent) were positive for a test result. These individuals were then matched to negative controls of the same age and ethnic group who were tested within the same week.
The time that passed since the second dose was completed increased the probability of positive results. For example, 1.3% of all participants tested positive for positive 21-89 days after a second dose. The percentage increased to 2.4% after 90-119, 4.6% after 120-149, 10.3% after 150-179 and 15.5 percent after 180 days.
After taking into account other potentially influential factors, the researchers found an increased risk of contracting an infection with the time that has passed since the second dose.
The risk of contracting the infection was 2.37-fold greater after 90-119 days than it was after the second dose. 2.66-fold greater after 120-149 days; 2.82-fold higher following 150-179 day; 2.822-fold higher thereafter; and 2.82-fold higher after 180 days or more.
Researchers acknowledge that their findings are limited and that they cannot exclude the possibility of other factors, such as household size, population density or virus strain, having an impact on their findings.
However, the large study of the people who had all received the vaccine was extensive and the researchers were able to analyze the data in depth which suggests that the results are reliable.
According to the researchers, the protection of individuals who received two doses Pfizer-BioNTech vaccine appeared to decrease over time and the risk for breakthrough infection increased progressively relative with the protection offered during the initial 90-day period.
The results suggest that consideration of a third dose of vaccine might be warranted They add.
Israel, A., et al. (2021) Elapsed time since BNT162b2 vaccine and SARS-CoV-2-related risk A test-negative design study. BMJ. doi.org/10.1136/bmj-2021-067873.
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