Covid-19 vaccine efficacy against Delta variant declines after three months

Covid-19 vaccines become less effective against the Delta variant three months after full vaccination, according to new research by the University of Oxford.

The results of the large UK study looked at how the Pfizer-BioNTech and AstraZeneca vaccines protect patients against Covid-19.

Probably you get some protection against more severe disease for a few years. The whole point of the vaccines is to stop people from getting ill and dying

Prof Paul Hunter,
University of East Anglia

Researchers found that 90 days after the second shot the vaccines' efficacy in preventing infections 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 over, than among younger case studies.

Scientists also found that when vaccinated people were infected with Delta, they had similar levels of virus in their bodies as those who had not had the injections.

This could imply that those who are vaccinated could still spread the virus. It also raises questions over whether herd immunity can be achieved even in countries of mostly vaccinated people.

Findings ‘not surprising’

Prof Paul Hunter, a professor in medicine and infectious diseases specialist at the University of East Anglia in the UK, said the results of the latest study were "not surprising".

He that while the vaccines' effectiveness at preventing mild infections starts to fall away in a matter of months, the shots continued to prevent severe disease for far longer.

Immunity against mild disease, from vaccination or previous infection, was, he said, "short-lived". But the immunity to the more severe viral pneumonia from Covid-19 that can result in hospital admission and death "lasts a lot longer".

"Probably you get some protection against more severe disease for a few years, so what this means is that the vaccines are not going to stop the spread of the virus. Most of us who work in the area have known this for at least six months," he said.

"Herd immunity is not and never was going to be achievable. The whole point of the vaccines is to stop people from getting ill and dying."

While the new study indicates a reduction in vaccine efficacy over time, Prof David Taylor, professor emeritus of pharmaceutical and public health policy at University College London, said the vaccines were performing "relatively well".

"The real [challenge] for the world is getting production up to the level so we can supply poor world [countries], which isn't happening at the moment," he said.

He said, however, that scientists and governments would look for enhanced vaccines that were more effective against the Delta variant and other variants, and that such "tweaked" shots could be used widely as boosters.

The UK survey, run by the University of Oxford and the Office for National Statistics, analysed more than three million PCR tests from a random sample of people.

The results are likely to encourage more countries to offer booster shots for fully vaccinated people. The policy is already in place in the UAE.

Health officials in the Emirates recommend that people have booster shots six months after their second vaccine dose, unless they are considered vulnerable, in which case they should have it after three months.

This week, US President Joe Biden announced that Americans who have had both doses of Pfizer-BioNTech or Moderna will be allowed a booster after eight months.

Israel this month started giving third doses of Pfizer-BioNTech to the elderly. Initial results show they have been 86 per cent effective for people aged 60 and over.