AstraZeneca vaccine more effective with delayed doses, says study

Immune response found to be greater among participants who received their shots 12 weeks apart

The immune response in people aged between 18 and 55 was at least twice as strong in the group having a longer delay between doses. AFP

A three-month interval between doses of the Oxford-AstraZeneca Covid vaccine is more effective than a six-week interval, a new study published in The Lancet suggests.

The study combined data from randomised, controlled trials of 17,178 people in Britain, Brazil and South Africa.

Participants were adults and received either two standard doses of the vaccine (8,597 participants) or a placebo (8,581).

A subset (1,396 people) in the UK trial received a lower first dose of the vaccine.

Participants who received their doses at least 12 weeks apart were found to be 81 per cent protected; those given their doses within six weeks were 55 per cent protected.

Efficacy results were supported by the immune response in people aged 18 to 55, which was at least twice as strong as that of people whose shots were spaced farther apart.

The report's authors concluded that lengthening the delay between doses may be beneficial when vaccine supplies are limited, implicitly backing the UK government's single-dose strategy that prompted concern among healthcare professionals.

In concert with its speedy procurement and ratification protocols, it is this hitherto unsanctioned extension that has enabled Britain to give shots to more people.

So far the UK has given administered 17.2 vaccine doses, or enough to fully immunise about an eighth of its 67 million people. Only the UAE and Israel have administered more doses per head of the population. The figures do not show the proportion of people who received both doses.

Its speedy vaccine programme has provided some welcome relief for the UK government, which has been roundly criticised for its response to the pandemic.

On Saturday, as Prime Minister Boris Johnson pledged that all UK adults would be immunised by August, the country reached 120,365 Covid fatalities, the fifth-highest death toll in the world.

“This latest analysis confirms our previous findings of the higher efficacy of a low- then standard-dose regimen,” said study author Dr Merryn Voysey from the University of Oxford.

“However, with additional data available, we have found that the enhanced efficacy and immunity may be partly driven by the longer interval between doses that was common in this trial group.

“This further supports the relationship we have found between vaccine interval and efficacy in those receiving two standard doses, which is the preferred regimen because there are more data to support its use, and because it is simpler to deliver a vaccine programme when the same vaccine is given for both doses.”

Covid transmission lessened by vaccine

The effect of vaccines on transmission was also studied in the trials.

The authors estimated that a single dose of the vaccine may lead to a 64 per cent reduction, and two doses a 50 per cent reduction.

“We saw a reduction in the overall number of positive cases which indicates that the vaccines may reduce infections,” said study lead Professor Andrew Pollard, also from the University of Oxford.

He said “real-world assessments” of vaccine efficacy would be required to confirm the “preliminary result”.