The risk of contagion for those vaccinated with Pfizer-BioNTech begins to increase 90 days after the second dose: this is demonstrated by a study on over 80,000 people carried out in Israel, one of the first countries in the world to launch mass vaccination against Covid-19. It was known that protection begins to decline after a few months from the second inoculation but the Israeli study – the country that first started immunization and has already started for some time with the third dose and a few days with that of children – confirm this.
The results, published in the British Medical Journal (Bmj) from the research institute of Leumit Health Services, highlight the usefulness of the third booster dose and provide valuable information on the timing for administration.
The analyzed sample
The study is based on the examination of electronic medical records of 83,057 adults (mean age 44 years) who were subjected to molecular swabs at least three weeks after the second dose of vaccine between May and September and who had never previously shown signs of infection with SarsCoV2.
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The results of the study
The results show that 7,973 participants (9.6% of the total) tested positive for the swab. The analysis shows that, after the second dose of the vaccine, the positivity rate increases over time: it is equal to 1.3% between 21 and 89 days, 2.4% between 90 and 119 days, 4.6% between 120 and 149 days, 10.3% between 150 and 179 days, finally 15.5% after 180 days.
Compared to the first 90 days after the second vaccine dose, the risk of infection is 2.37 times higher after 90-119 days, 2.66 times higher after 120-149 days and 2.82 times higher after 150 days .
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The limits of the study
The researchers acknowledge that the interpretation of the data is limited by the observational design of the study and that the influence of other factors not considered in the analysis such as virus strain, number of family members and population density cannot be excluded.
In any case, the study was conducted on a large number of people who received the same vaccine, so the results are robust enough to conclude that the protection induced by the two vaccine doses of Pfizer-BioNTech decreases over time and the risk of contagion it gradually increases after the first three months.
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