The AstraZeneca-Oxford University Covid-19 vaccine is more effective with a 12-week interval between the first and second doses, according to research.
The study, published in The Lancet medical journal on Friday, found that a 12-week gap between doses led to 81 per cent protection against serious illness from coronavirus infection, compared with 55 per cent protection for a six-week interval between the two doses.
The findings support the UK government’s decision to extend the window to three months between doses to allow more people to receive some level of protection. The World Health Organisation earlier this month welcomed the move.
The 12-week window is cited as a reason for Britain's success with its vaccination drive in comparison to other nations.
The UK has vaccinated more than 16.4 million people with a single, behind only Israel and the UAE in inoculation per capita, government statistics show.
According to the study of more than 17,000 people, a single dose of the vaccine offers 76 per cent protection in the first three months, increasing to 81 per cent after a second dose.
Prof Andrew Pollard, chief investigator of the Oxford Vaccine Group and lead author of the study, said the 12-week gap can “achieve the greatest public health benefit”.
“Where there is a limited supply, policies of initially vaccinating more people with a single dose may provide greater immediate population protection than vaccinating half the number of people with two doses,” he said.
“In the long term, a second dose should ensure long-lived immunity, and so we encourage everyone who has had their first vaccine to ensure they receive both doses.”
Faced with a resurgence in infections and new, highly transmissible variants of the virus, many countries are hoping to broaden immunisation by giving some protection to as many people as possible with a first dose, while delaying subsequent shots.
In the 22 days after the study’s participants received a first dose of the vaccine, no one was admitted to hospital, while 15 people were among those who received a placebo. A total of 8,597 received the vaccine and 8,581 were injected with a placebo.
While previous studies showed vaccines can prevent serious illness or death, there is limited data to suggest they reduce Covid-19 infection.
But Prof Pollard estimated that a single dose of the vaccine could reduce transmission by up to 64 per cent, with those who received the vaccine in the study less likely to test positive for the virus.
“If the vaccine had no impact on transmission, we would expect that the number of positive tests in our trial would be the same in vaccine and control groups,” he said.
“This is because the vaccine would convert severe cases to mild cases, and mild cases to asymptomatic cases. However, we saw a reduction in the overall number of positive cases, which indicates that the vaccines may reduce infections.”
According to "real world" data handed to the British government, vaccines appear to be playing a part in reducing coronavirus transmission by up to two-thirds.
UK Prime Minister Boris Johnson will consider the data showing the effects of one dose of either the Oxford-AstraZeneca or Pfizer-BioNTech vaccines as he finalises a route out of lockdown next Monday.
The Telegraph quoted government sources as saying that the Public Health England data – which is not yet publicly available – was "very encouraging".
Prof Neil Ferguson, a key scientific adviser to the government, said the two thirds drop in transmission was “not too far off” the modelling he has seen.
“There are two factors we have to look at: one is how quickly our infection levels are declining, in particular how quickly hospitalisations and deaths are declining. The second is the picture of the real world effectiveness of vaccines,” he told the BBC on Friday.
“They are both looking promising at the moment.”
Prof Adam Finn from the University of Bristol said “everything is moving in the right direction”.
“We’ve got to the point in our study in Bristol to say it’s definitely having an effect – it’s just hard to put an exact number on it at the moment,” he said.
“It’s becoming clear for the Pfizer-BioNTech vaccine, which we’ve been using for a month longer, and it will take slightly longer for the Oxford-AstraZeneca vaccine to come through, but they are definitely doing the job.”