A study conducted by Israel's Leumit Health Servisses Research Institute has identified an increased risk of new coronavirus infections beginning 90 days after taking the second dose of Pfizer/BioNTech vaccines.

Thus, the results may justify the possibility of administering a third dose.

Despite this, the study published in The BMJ, acknowledges that the vaccine in reference guaranteed efficient protection only in the first few weeks after vaccination, with immunization weakening in some individuals over time.

The researchers examined the electronic health records of 80,057 adults (with a mean age of 44) who had undergone a PCR test at least three weeks after the second dose and who had no evidence of previous covid-19 infection.

Of these participants, 7,973 (9.6%) tested positive - these individuals were paired with negative controls of the same age and ethnic group who underwent the test 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 number increased to 2.4% after 90-119 days, to 4.6% after 120-149 days, to 10.3% after 150-179 days, and to 15.5% after 180 days or more.

The researchers recognize that interpretation of their results is limited by the observational design and cannot rule out the possibility that other factors not evaluated, such as sample size, population density, or virus variant, may have contributed to the effect.

Source: Lusa