A single dose of the Pfizer vaccine appears to cut the risk of Covid transmission by three quarters, research by Cambridge University suggests.
Scientists said the findings were "dramatic" and showed that vaccines offered a way out of lockdown and "a much brighter future".
The study involved almost 9,000 hospital workers who were screened for Covid in January, meaning that cases could be detected whether or not symptoms were showing.
It found that 0.8 per cent of tests from unvaccinated workers were positive, compared with just 0.2 per cent of those who had been vaccinated at least 12 days before. A similar reduction was seen among those with symptoms of the virus when people given the jab were compared with those who had yet to receive it.
But the impact shown on asymptomatic cases is particularly significant because it suggests the vaccines are blocking virus transmission.
Dr Mike Weekes, an infectious disease specialist at CUH and the University of Cambridge's Department of Medicine, who led the study, said: "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.
"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. We still need social distancing, masks, hand hygiene and regular testing until the pandemic is under much better control."
Dr Nick Jones, the first author on the study and an infectious diseases registrar at Cambridge University Hospital said: "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.
"This is fantastic news for both hospital staff and patients, who can be reassured that the current mass vaccination strategy is protecting against asymptomatic carriage of the virus in addition to symptomatic disease, thereby making hospitals even safer places to be."
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."
The findings, which have yet to be peer reviewed, follow a host of positive findings on the effectiveness of the vaccines. Earlier this week, data from Scotland showed that one dose of the AstraZeneca jab reduced hospitalisations by 94 percent, with a drop of 85 per cent for those given the Pfizer jab.
Research from Public Health England (PHE) involving healthcare workers showed that one dose of Pfizer was enough to cut infections by between 57 and 70 per cent. PHE's experts said it was likely to mean a similar impact on transmissions, and that early data on the Astra Zeneca jab showed the same trends.
The new study by a team at Cambridge University Hospitals NHS Foundation Trust and the University of Cambridge analysed results from thousands of Covid tests carried out each week as part of its screening programmes on hospital staff who showed no signs of infection.
Vaccination for healthcare workers on the Cambridge University Hospitals site began on Dec 8, with mass vaccination from Jan 8. During a two-week period between Jan 18 and 31, the team screened similar numbers of vaccinated and unvaccinated staff using around 4,400 PCR tests per week.
When the team included symptomatic healthcare workers their analyses showed similar reductions, with 1.71 percent of unvaccinated healthcare workers testing positive compared with 0·40 per cent of those who had received a jab at least 12 days before.