The results of the study were published in the journal BMJ.
The study was conducted by the Research Institute of Leumit Health Services in Israel. Israel was one of the first countries to introduce an extensive covid-19 vaccination campaign in December 2020, but infections have increased since June 2021.
The results confirm that the Pfizer-BioNTech vaccine provided excellent protection in the first weeks after vaccination, but suggest that the protection of some individuals deteriorates over time.
Extensive covid-19 vaccination campaigns around the world are helping to control the spread of the virus, but even in countries with high vaccination rates, breakthrough infections can occur, scientists believe are due to the gradual loss of immunity over time.
Examination of the time elapsed since vaccination and the risk of infection may provide important clues as to the need for a third injection and its recommended timing.
To this end, the researchers looked at electronic health records from 80,057 adults (mean age 44 years) who received a PCR test at least three weeks after the second injection and who had no evidence of previous covid-19 infection.
Of these 80,057 participants, 7,973 (9.6 percent) had a positive test result. These individuals were then pooled with negative controls of the same age and ethnic group and tested in the same week.
The number of positive results increased over time after the second dose. For example, in all age groups, 1.3 percent of participants gave a positive result 21–89 days after the second dose, but this rose to 2.4 percent after 90–119 days; 4.6 percent after 120-149 days; 10.3 percent after 150-179 days; and 15.5 percent after 180 days or more.
And after considering other potential contributing factors, the researchers found a significantly increased risk of infection over time after the second dose.
Compared to the first 90 days after the second dose, the risk of infection was 2.37-fold higher in all age groups after 90 to 119 days; 2.66 times higher after 120-149 days; 2.82 times higher after 150-179 days; and 2.82 times higher after 180 days or more.
Researchers acknowledge that the design of findings limits the interpretation of their findings, and they cannot rule out the possibility that other unmeasured factors, such as household size, population density, or virus stock, may have contributed.
However, this was an extensive study of people who all received the same vaccine, and the researchers were able to perform a detailed analysis of the data, suggesting that the results are strong.
As such, they conclude that in subjects who received two doses of Pfizer-BioNTech, protection appeared to diminish over time and the risk of a breakthrough infection gradually increased compared to protection during the first 90 days.
The results suggest that consideration of a third dose of the vaccine may be warranted, they added.
Source: The Nordic Page