The study says that the risk of infection with the Covid virus in cases of double stabbing increases over time

Experts who track the effectiveness of the Pfizer/BioNTech vaccine have found that the risk of contracting a Covid infection increases the longer since people have taken a second dose.

The study supports the need for booster doses to prevent breakthrough infections in people with diminished immunity, according to the team who published their findings in the British Medical Journal (BMJ).

Experts from the Leumit Health Services Research Institute in Israel found a gradual increase in the risk of contracting Covid-19 infection from 90 days after people received a 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 it has been fighting a resurgence of infections and looking closely at waning immunity.

Experts examined the electronic health records of 80,057 adults — with an average age of 44 — who had a PCR test 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 a cohort 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 in the same week.

The study found that the rate of positive results was found to rise in line with the 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 proportion 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-fold after 150-179 days; and 2.82-fold after 180 days or more.

While the study is of an observational nature, experts said it appears clear that immunity wanes after the first three months of double vaccination.

They said: “Large-scale vaccination campaigns for Covid-19 can achieve control of the spread of the virus, but even in countries with high vaccination rates, breakthrough infections can occur.

“The risk of developing (Covid) infection in adults who received two vaccine injections increased with time since vaccination compared to the reference (individuals vaccinated in the last 90 days).”

AstraZeneca said the antibody protection of its vaccine is also waning, although its chief executive, Pascal Soriot, said on Tuesday he believes the vaccine provides long-term T-cell immunity to older adults, which could be better than other vaccines.

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