A British study in the field of public health concluded that the protection provided by the two most widely used vaccines to prevent the mutated Delta strain of the Corona virus, which is currently prevalent, weakens within 3 months.

The study also concluded that those who were infected after receiving two doses of the Pfizer-Biontech or AstraZeneca vaccine may pose a greater risk to others than was the case with previous strains of the virus.

The study conducted by the University of Oxford, and based on more than 3 million swab from the nose or throat across Britain, found that 90 days after the second dose of the Pfizer or AstraZeneca vaccine, the efficiency of the first in preventing infection decreased by 75 percent and the second 61 percent.

This represented a decrease of 85 percent for Pfizer and 68 percent for AstraZeneca, which were observed two weeks after the second dose. The decline in effectiveness was more pronounced in those over the age of 35 years.

“Both vaccines, after two doses, still worked really well against delta,” said Sarah Walker, professor of medical statistics at Oxford who led the study.

The researchers did not specify the extent to which protection decreased over time, but they indicated that the efficiency of the two vaccines that were studied will converge after 4 to 5 months after the second dose.

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Highlighting the increased risk of infection from the delta strain, the study also showed that those who become infected despite being fully vaccinated tend to have a viral load similar to those without the vaccine, a clear regression from when the alpha strain was still prevalent in Britain.

The results of the Oxford study are consistent with an analysis conducted by the US Centers for Disease Control and Prevention, as they come as the US government plans to make a third booster dose of vaccines widely available next month with the increase in infections with the Delta strain. She attributed this to data indicating a decline in protection from the vaccine over time.

Israel began vaccinating with third doses of the Pfizer vaccine last month to counter a rise in local infections caused by the Delta strain.

Several European countries are also expected to start providing booster doses for the elderly and those with weakened immune systems.

Pfizer said the effectiveness of its vaccine declines over time.

AstraZeneca said last month that it was still studying how long its vaccine would protect and whether a booster dose would be needed to maintain immunization against the virus.

“The fact that we’re seeing … a larger viral load suggests (…) that herd immunity may actually become more difficult,” said study co-author Quinn Boyles, also from Oxford University.

Herd immunity occurs when a large enough section of the population is immune to a pathogen, either through vaccination or previous infection, which stops the growth of infected numbers.