Longer gap between AstraZeneca vaccines leads to better immune response, study finds

New research also highlights possible benefits of a third dose of the Covid-19 jab

The more time between the administering of the two doses of AstraZeneca’s Covid-19 vaccine the better for immune responses in the recipient, new research has confirmed, indicating the earlier approach taken by the Government in this regard could possibly have better results.

At the beginning of June, the gap between the two doses of the AstraZeneca vaccine was cleared for reduction from 12 weeks to eight weeks under updated advice from chief medical officer Dr Tony Holohan.

This advice was submitted on foot of a fresh recommendation from the National Immunisation Advisory Committee (Niac). The initial gap between the two doses was 16 weeks.

Previous UK studies have shown that one dose of the AstraZeneca vaccine is only 33 per cent effective in protecting against infection by the Delta Covid-19 variant, but that this rises to 60 per cent with a second dose.


However, soon-to-be-published, or preprint, research from the University of Oxford has found a greater level of antibody production in general in those recipients who waited for longer durations between the AstraZeneca jabs.

“A long extension of the dose interval (up to 45 weeks) between the first and second dose further enhances the immune response to the second dose, when compared with shorter dose intervals,” the study found.

The paper, which has not yet been peer reviewed, examines “the persistence of immunogenicity”, or vaccine effectiveness over time, of a single dose and an extended interval between first and second doses, and of the body’s response to a third dose as a late booster.

It found the 90 participants in the study who received a third dose showed antibody levels that were “significantly higher” when compared with the response 28 days after a second dose.

Study limits

Trinity College Dublin professor of experimental immunology Kingston Mills noted, however, that the study was somewhat limited given it looked at all antibody production rather than zeroing in on the more relevant neutralising antibodies, those that defend cells.

However, he said it indicated a reduction in the amount of time between the first two doses “will reduce rather than enhance the effectiveness of the vaccine, [or] is likely to.

“Based on this it would suggest that you are not going to get better protection by speeding it up. If anything, you are going to get worse protection.”

The Oxford researchers argue that, in the context of countries challenged by vaccine supply, a single AstraZeneca dose with a second given after a prolonged period may prove an effective strategy.

Sam McConkey, professor of infectious diseases at the Royal College of Surgeons in Ireland, did not see the research but said its reported findings appeared to underscore what was already known.

He said, however, it prompted further debate on the AstraZeneca jab: namely, on whether it should be boosted by other vaccines and whether it should be made available to younger cohorts based on availability.

“It’s almost certain that we are going to have a big outbreak of the Delta variant in the next couple of months,” he said, suggesting AstraZeneca could play a role in limiting that.

Dr Peter English, a former consultant in communicable disease and recent chair of the British Medical Association's Public Health Medicine Committee, said it was a rule of thumb that "while you risk reducing vaccine efficacy if you give booster doses sooner than recommended, you can extend the prime-boost interval almost indefinitely . . . without a reduction in the efficacy of the booster".

He said the finding of increased antibody responses was “entirely as expected” when the second dose was delayed.

Mark Hilliard

Mark Hilliard

Mark Hilliard is a reporter with The Irish Times