A single dose of Pfizer and BioNtech's Covid-19 vaccine cuts the number of asymptomatic infections and could significantly reduce the risk of transmission of the virus, results of a UK study found on Friday.
Researchers analysed results from thousands of Covid-19 tests carried out each week as part of hospital screenings of healthcare staff in Cambridge, eastern England.
"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, an infectious diseases specialist at Cambridge University Hospital, who co-led the study.
After separating the test results from unvaccinated and vaccinated staff, Jones' team found that 0.80% tests from unvaccinated healthcare workers were positive.
This compared with 0.37% of tests from staff less than 12 days post-vaccination - when the vaccine's protective effect is not yet fully established - and 0.20% 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.
This suggests a four-fold decrease in the risk of asymptomatic Covid-19 infection amongst healthcare workers who have been vaccinated for more than 12 days, and 75% protection, said Mike Weekes, an infectious disease specialist at Cambridge University's department of medicine, who co-led the study.
The level of asymptomatic infection was also halved in those vaccinated for less than 12 days, he said.
Britain has been rolling out vaccinations with both the Pfizer Covid-19 shot and one from AstraZeneca since late December 2020.
"This is great news – 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," Weeks said. "But we have to remember that the vaccine doesn't give complete protection for everyone."
Key real-world data published on Wednesday from Israel, which has conducted one of the world's fastest rollouts of Pfizer's Covid-19 vaccine, showed that two doses of the Pfizer shot cut symptomatic Covid-19 cases by 94% across all age groups, and severe illnesses by nearly as much.