The findings from Addenbrooke’s Hospital in Cambridge indicated 75-per-cent protection from Covid-19.
The results also point to a four-fold decrease in the risk of asymptomatic Covid infection among healthcare workers who have been vaccinated for more than 12 days – suggesting the first dose will significantly reduce the spread of the virus.
“Our findings show a dramatic reduction in the rate of positive screening tests among asymptomatic healthcare workers after a single dose of the Pfizer-BioNTech vaccine,” said Nick Jones, a Cambridge University Hospital specialist who co-led the study.
The UK has been rolling out Covid vaccinations since late December 2020, with both the Pfizer shot and the Oxford-AstraZeneca jab given to people in high-risk categories.
As part of their study on the Pfizer vaccine, Cambridge researchers analysed results from thousands of Covid tests carried out each week as part of hospital screenings of healthcare staff.
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Dr Mike Weekes, an infectious disease specialist at Cambridge University’s department of medicine, who co-led the study, hailed the findings as “great news”.
He said: “The Pfizer vaccine not only provides protection against becoming ill from SARS-CoV-2 but also helps prevent infection, reducing the potential for the virus to be passed on to others.”
Dr Weekes added: “This will be welcome news as we begin to plot a roadmap out of the lockdown, but we have to remember that the vaccine doesn’t give complete protection for everyone.”
After separating the test results from unvaccinated and vaccinated staff, the Cambridge team found that 0.8 per cent of tests from unvaccinated healthcare workers were positive.
This compared with 0.37 per cent of tests from staff less than 12 days after vaccination – when the vaccine’s protective effect is not yet been fully established – and 0.2 per cent of tests from staff at 12 days or more post-vaccination.
The study and its results have yet to be independently peer-reviewed by other scientists, but were published online as a preprint on Friday.
Independent scientists said the findings were very encouraging.
Jonathan Ball, professor of molecular virology at the University of Nottingham, said: “To see such a reduction in infection rates after a single dose of the Pfizer vaccine is very impressive, and shows that vaccination truly does offer a way out of the current restrictions and a much brighter future.
“It will be important to understand whether the reduced risk of infection played out across all the exposure risk groups included in the study, but nonetheless, this is still excellent news.”
Dr Andrew Freedman, of the Cardiff University School of Medicine, said the latest study “demonstrates clearly the effectiveness of the vaccine in preventing infection”. He added: “This means that vaccination will lead to a substantial reduction in transmission of the virus.”
Key real-world data published on Wednesday from Israel, which has conducted one of the world’s fastest rollouts of Pfizer’s vaccine, showed that two doses of the Pfizer shot cut symptomatic Covid-19 cases by 94 per cent across all age groups, and severe illnesses by nearly as much.
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