There had been concerns that in countries with a low supply of vaccines delays beyond the recommended eight to 12 week gap between doses would result in less overall protection from Covid.
But a pre-print study by the team behind the UK’s home-grown vaccine said there was a “heightened immune response” to the virus up to 45 weeks between doses.
Professor Sir Andrew Pollard said there was still a “trade off” to consider in delaying the second dose, as a single dose offered far lower protection than two doses.
But he said the significance of the research was that it showed protection from a first dose doesn’t “suddenly fall off a cliff” after three months.
Sir Andrew said: “If you have a lot of transmission in a population, then if you have got plenty of supply… you wouldn’t wait a year for a second dose.”
He said the high level of protection after two doses meant that it was still unclear whether a third booster dose would be needed, though he said it was “critical” that preparations continued to be put in place as a precaution.
The study found that a third dose given more than six months after the second dose leads to a substantial increase in antibodies and induces a strong boost to immune response against Covid, including variants.
It said that a longer delay between first and second doses “may be beneficial”, resulting in the creation of more antibodies and an enhanced immune response after the second dose.
Sir Andrew, professor of paediatric infection and immunity and lead investigator of the Oxford trial, said: “This should come as reassuring news to countries with lower supplies of the vaccine, who may be concerned about delays in providing second doses to their populations. There is an excellent response to a second dose, even after a 10 month delay from the first.”
Associate professor Teresa Lambe, lead senior author, said: “It is not known if booster jabs will be needed due to waning immunity or to augment immunity against variants of concern.
“We show that a third dose of [AstraZeneca] is well tolerated and significantly boosts the antibody response. This is very encouraging news, if we find that a third dose is needed.”