A single dose of a Covid-19 vaccine can cut the spread of the virus in households by up to half, a British study shows.
The Public Health England research offers insight into one of the big unknowns over Covid-19 vaccinations – the extent to which they prevent transmission of the coronavirus .
The results could strengthen the case for Prime Minister Boris Johnson's plans to end England's lockdown in June.
But scientists warned against making a "big mistake" by dropping Covid-19 restrictions too early based mainly on the success of vaccines, while former prime minister Tony Blair's think tank recommended inoculating adolescents and delaying the UK's final step out of lockdown until after summer.
People who received a first dose of vaccines developed by either AstraZeneca or Pfizer – and became infected three weeks later – were between 38 per cent and 49 per cent less likely to pass the disease on to those in their household compared with those who were unvaccinated.
The study showed a single shot could stop a vaccinated person developing symptomatic infection initially, and the risk reduced by 60 per cent to 65 per cent from four weeks after one dose of either vaccine.
Dr Mary Ramsay, head of immunisation at PHE, said the study showed vaccines were vital for “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,” she said.
Prof Peter Openshaw from Imperial College London said the study's results were "very reassuring" but warned the UK it could not rely solely on vaccines to reduce the spread of the virus.
"We mustn't drop our guard in the summer – we must use the summer to strengthen our precautions and to roll out vaccines into the groups which are most transmitting, which is younger people," he told the BBC Radio 4 Today programme on Wednesday.
"We’re doing the right thing by being cautious and not easing too fast. Easing too fast will be a big mistake, we don’t want to lose all the ground we’ve gained by massive public effort."
A new report from the Tony Blair Institute on Wednesday warned the UK's final lockdown easing on June 21 – when most social distancing restrictions are lifted –could overtake the vaccination schedule.
The think tank recommended delaying step four of the roadmap by a month to allow adolescents to be vaccinated.
In the US, people aged 16 and older are eligible for a vaccine but in the UK it is limited to over-18s.
"On its own, delaying step four could leave the country under-protected for a recurrence later in the year as the virus may spread more readily in winter," the institute said.
"Offering a vaccine to a further 6.8 per cent of the population could be critical in taking population protection levels over the winter herd immunity threshold, thereby preventing a further wave of infections in the new year."
Health Secretary Matt Hancock emphasised the importance of the vaccination programme while addressing the PHE study.
"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," he said.
The study included more than 57,000 contacts from 24,000 households in which there was a lab-confirmed case that had received a vaccination, researchers said, compared with nearly 1 million contacts of unvaccinated cases.
Britain's Covid-19 vaccination campaign is one of the world's fastest, with more than 33.8 million first doses given and a quarter of adults receiving two.
That has generated real-world data on how the Pfizer and AstraZeneca drugs work outside of clinical trials, and health officials said this month the distribution of vaccines had prevented more than 10,000 deaths of people aged 60 and older by the end of March.
Previous data that showed effectiveness of vaccines in older people helped to inform other countries that reversed age limits on AstraZeneca's shot.
