Covid-19: study found that 90 days after second vaccine risk of infection gradually increases

A study confirms a gradual increase in the risk of COVID-19 infection from 90 days after receiving a second dose of the Pfizer/BioNTech vaccine.The results suggest that the consideration of a third dose may be justified, say those responsible for this work published in the journal The BMJ. The study from the Leumit Health Services Research Institute in Israel confirms that the Pfizer/BioNTech vaccine provided “excellent protection” in the first few weeks after vaccination, but suggests that vaccination decreases for some individuals over time. Examining the time since vaccination and the risk of infection could provide important clues about the need for a third injection and the best time for it, a journal statement explains. The researchers examined the electronic health records of 80,057 adults (with an average age of 44 years), who had undergone a PCR test at least three weeks after their second injection, and who had no indication of a previous COVID-19 infection. Of these 80,057 participants, 7,973 (9.6%) had a positive test result; these individuals were paired with negative controls of the same age and ethnicity who were tested in the same week. According to the study, the rate of positive results increased with the time elapsed since the second dose. For example, in all age groups, 1.3% of participants tested positive between 21 and 89 days after the second dose, but this figure 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. After taking into account other potentially influential factors, the scientists found “a significantly increased risk” of infection with the time elapsed since a second dose, the note concludes. The researchers acknowledge that interpretation of their results is limited by observational design and cannot rule out the possibility that other unmeasured factors, such as household size, population density, or variant virus, may have had an effect. However, this is a large study of people who received the same vaccine and a detailed analysis of the data could be performed, “suggesting that the results are robust.”

Original source in Spanish