Pfizer and AstraZeneca vaccines less effective versus Delta, large UK study finds

According to the survey, which was conducted by the University of Oxford and the Office for National Statistics, the effectiveness of the Pfizer-BioNTech shot against "high viral burden infections" fell from 92.0% 14 days after the second dose was given to 78.0% 90 days afterwards.

In the case of the AstraZeneca shot the drop was smaller, from 69.0% to 61.0%.

Furthermore, the viral load in those infected with the virus was similar regardless of whether they had been inoculated or not.

For some observers, those findings suggested that vaccinating a large swathe of the population might not protect those who weren't administered a vaccine, casting doubt on the ability to reach what is known as 'herd immunity'.

Over 3.0m random PCR tests were analysed as part of the survey.

Data on vaccines' ability to protect against hospitalisation and severe cases as time elapsed was not yet available.

However, initial results from Israel revealed that providing a third dose of Pfizer-BioNTech boosted vaccines' effectiveness to 86.0% in those aged 60 or over.

Also, on 27 July, the US Centers for Disease Control and Prevention said its data showed that viral loads among infected people, who had been vaccinated, declined more quickly than in those who had not been given a shot.

"This means fully vaccinated people are likely infectious for less time than unvaccinated people."

At the time, the CDC also said that vaccines continued to offer "strong" protection against serious illness and death to those who became infected despite having been inoculated.