A single dose of a Covid-19 vaccine can slash transmission by up to half, according to a new study from Public Health England (PHE).
The breakthrough findings offer further hope that the pandemic can be brought under control as vaccinated people are far less likely to pass the virus onto others.
The new study found that people given a single dose of either the Pfizer/BioNTech or Oxford/AstraZeneca vaccines – and who became infected at least three weeks later – were between 38% and 49% less likely to pass the virus on to people living in their homes, compared to those who were unvaccinated.
Protection was seen from around 14 days after vaccination, with similar levels regardless of a person’s age.
Other studies have already shown that both vaccines are highly effective at stopping people getting sick and ending up in hospital.
Experts will now assess whether two doses of vaccine can cut transmission of the virus even further, and more work is being carried out on transmission in the general population.
PHE said similar results could be expected in places where the risk of transmission is similar to the home, such as shared accommodation and prisons.
The study included data from January and February, when the Kent strain was dominant in the UK.
Health and Social Care Secretary Matt Hancock said: “This is terrific news – we already know vaccines save lives and this study is the most comprehensive real-world data showing they also cut transmission of this deadly virus.
“It further reinforces that vaccines are the best way out of this pandemic as they protect you and they may prevent you from unknowingly infecting someone in your household.
“I urge everybody to get their vaccines as soon as they are eligible and make sure you get your second dose for the strongest possible protection.
“This is a huge national effort and we will beat the virus together.”
The study, which has yet to be peer-reviewed, included over 57,000 people living in 24,000 households who were the contacts of a vaccinated person.
They were compared with nearly one million contacts of people who had not had a vaccine.
Contacts were defined as secondary cases of coronavirus if they tested positive two to 14 days after the initial household case.
Most of the people in the study were under the age of 60.
Dr Mary Ramsay, head of immunisation at PHE, said: “Vaccines are vital in helping us return to a normal way of life.
“Not only do vaccines reduce the severity of illness and prevent hundreds of deaths every day, we now see they also have an additional impact on reducing the chance of passing Covid-19 on to others.
“I encourage anyone who is offered a vaccine to take it as soon as possible.
“While these findings are very encouraging, even if you have been vaccinated, it is really important that you continue to act like you have the virus, practise good hand hygiene and follow social distancing guidance.”
Dr Peter English, a retired consultant in communicable disease control, said the study was robust and the authors may have in fact underestimated the effect of vaccines on transmission.
He added: “These findings are really important. They add to our reasons to hope that the vaccines will truly add to herd immunity.
“The evidence was already mounting that vaccination will prevent people from becoming infected (and if they aren’t infected, they can’t transmit the infection).
“This study shows that even if people who are vaccinated do become infected, they are considerably less likely to be infectious, and to pass the infection on to others.
“This is an extremely encouraging set of findings.”
The Pfizer and AstraZeneca vaccines are credited with having saved 10,400 lives among the over-60s as of the end of March.
Data out last week from the national Covid-19 Infection Survey run by the University of Oxford and the Office for National Statistics (ONS) also found that vaccines are likely to cut transmission.
Just one dose of either the Pfizer BioNTech or AstraZeneca vaccines cut coronavirus cases by two-thirds and were 74% effective against symptomatic infection, according to the real-world UK data.
After two doses of Pfizer, there was a 70% reduction in all cases and a 90% drop in symptomatic cases – these are the people who are most likely to transmit coronavirus to others.