Experts tracking the effectiveness of the Pfizer/BioNTech vaccine have found that the risk of contracting a Covid infection increases longer since people take the second dose.
The study supports the need for booster doses to prevent breakthrough infections in people with reduced immunity, according to the team, who published their findings in the British Medical Journal (BMJ).
Experts from the Research Institute of Leumit Health Services in Israel found a gradual increase in the risk of contracting a Covid-19 infection from 90 days after people received the second dose of the Pfizer/BioNTech vaccine.
Israel was one of the first countries to launch a massive vaccination campaign against the Covid-19 virus in December 2020, but has been battling a resurgence of infections and is closely watching dwindling immunity.
Experts examined the electronic health records of 80,057 adults (with a mean age of 44 years) who underwent PCR testing between mid-May and September at least three weeks after the second vaccine injection.
None of the people in the sample had evidence of previous infection with Covid-19.
Of the group of 80,057 people, 7,973 (9.6%) had a positive test result.
They were then matched with people of the same age and ethnic group who tested negative that same week.
The study found that the rate of positive outcomes was found to increase with time since people took their second dose.
Across all age groups, 1.3% of subjects tested positive 21–89 days after the second dose, but this percentage 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.
Compared with the first 90 days after the second dose, the risk of infection in all age groups was 2.37-fold higher after 90-119 days; 2.66-fold after 120-149 days; 2.82 times after 150-179 days; and 2.82 times after 180 days or more.
While the study was observational, experts say it seems clear that immunity diminishes after the first three months of double vaccination.
They said: “Large-scale vaccination campaigns for Covid-19 can control the spread of the virus, but even in countries with high vaccination rates, supernatural infections can occur.
“The risk of developing a (Covid) infection in adults who received two vaccine shots increased over time since vaccination compared to reference (individuals vaccinated in the last 90 days).
AstraZeneca said the antibody protection from its vaccine was also reduced, although its chief executive, Pascal Soriot, said Tuesday he believes the vaccine provides older adults with long-term T-cell immunity, which could be better than other vaccines.
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