Pfizer and Astra vaccines less effective after 90 days, says study

The Oxford University study found that 90 days after a second shot of the Pfizer or Astra vaccine, their efficacy in preventing infections had slipped to 75 per cent and 61 per cent, respectively

A British public health study has found that protection from either of the two most commonly used Covid-19 vaccines against the now prevalent Delta variant of the coronavirus weakens within three months.
 
It also found that those who get infected after receiving two shots of either the Pfizer-BioNTech or the AstraZeneca vaccine may be of greater risk to others than under previous variants of the coronavirus. Based on more than three million nose and throat swabs taken across Britain, the Oxford University study found that 90 days after a second shot of the Pfizer or Astrazeneca vaccine, their efficacy in preventing infections had slipped to 75 per cent and 61 per cent respectively.

 
That was down from 85 per cent and 68 per cent, respectively, seen two weeks after a second dose. The decline in efficacy was more pronounced among those aged 35 years and older than those below that age. “Both of these vaccines, at two doses, are still doing really well against Delta... When you start very, very high, you got a long way to go,” said Sarah Walker, an Oxford professor of medical statistics and chief investigator for the survey.
 

Walker was not involved in work on AstraZeneca's vaccine, which was initially developed by immunology experts at Oxford. The researchers would not project how much more the protection would drop over time, but suggested that the efficacy of the two vaccines studied would converge within 4-5 months after the second shot.

 
Highlighting the increased risk of contagion from the Delta variant, the study also showed that those who do get infected despite being fully vaccinated tend to have a viral load similar to the unvaccinated with an infection, a clear deterioration from when the Alpha variant was still dominant in Britain.
 
The Oxford findings are in line with an analysis by the US Centers for Disease Control and Prevention  and come as the US government outlines plans to make Covid-19 vaccine booster shots widely available next month amid a rise in Delta variant infections.