The Delta variant has significantly reduced the effectiveness of the UK’s COVID vaccination programme, according to the findings of a major new study.
One expert said the findings highlighted the need to press on rapidly with a planned booster campaign.
Oxford University researchers say that unvaccinated people now have limited protection against infection – because vaccinated people can spread the virus.
Double vaccination in itself is now only giving previously uninfected people the same level of protection as someone who has been through infection, they found.
The research found the effectiveness of the Pfizer mRNA vaccine declines rapidly in the face of Delta infection – although initially it gives much more protection than the Oxford adenoviral vaccine.
They also found that young recipients of vaccination gain more protection than older people.
The findings come as the summer holiday lull in infection rates seemed to be fading out – with daily reported infections reaching their highest level in nearly a month.
Yesterday the UK reported 33,904 new cases of infection and 111 new deaths from the virus. On Tuesday, the UK reported 170 new deaths, the largest number since early March.
English hospitals were treating 5,514 infected patients yesterday, the largest number since mid-March, and a 10% increase in one week.
The latest study is based on an analysis of infections across the UK in 2021. This includes test results from nearly 360,000 people between May and July when the Delta variant has been dominant.
This showed that people with previous COVID disease were 73% less likely than unvaccinated people to contract infection. 14 days after a second vaccination, those who received the Oxford vaccine were 67% less likely to contract infection – while those receiving the Pfizer vaccine were 82% less likely than the unvaccinated to become infected.
However, three months after the second vaccination, people who contracted the virus suffered high viral loads, regardless of which vaccine they received, the researchers report.
Researcher Professor Sarah Walker said: “We don’t yet know how much transmission can happen from people who get COVID-19 after being vaccinated – for example, they may have high levels of virus for shorter periods of time.
“But the fact that they can have high levels of virus suggests that people who aren’t yet vaccinated may not be as protected from the Delta variant as we hoped. This means it is essential for as many people as possible to get vaccinated – both in the UK and worldwide.”
Fellow researcher Dr Koen Pouwels said: “The fact that we did not see any effect of the interval between first and second doses, and the greater effectiveness of having had two doses, rather than one dose, supports the decision to reduce this to eight weeks now Delta is the main variant of concern in the UK.”
Commenting on the results, Dr Simon Clarke, a cellular microbiologist at the University of Reading, said: “We’re seeing here the real world data of how two vaccines are performing, rather than clinical trial data, and the data sets all show how the Delta variant has blunted the effectiveness of both the Pfizer and AstraZeneca jabs.
“On this evidence, it certainly supports the case for third booster jabs for vulnerable individuals, as is now happening in Israel. Moreover, these datasets scotch claims that protection provided by the UK’s vaccination programme is likely to be superior to Israel’s because of our longer window between first and second doses.”
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