Researchers said on Monday that a third jab of the Oxford-AstraZeneca COVID-19 vaccine produces a strong immune response, adding there was not yet evidence that such doses were needed, especially given shortages in some countries.

The Oxford University study found that a third shot of the vaccine increases antibody and T-cell immune responses, while the second dose can be delayed up to 45 weeks and also lead to an improved immune response.

The British government has said it is seeking plans for a fall vaccine booster campaign, with three-fifths of adults already having received both shots of a COVID vaccine.

Andrew Pollard, director of the Oxford Vaccine Group, said that evidence that the vaccine protects against current variants for a sustained period of time meant that such a booster may not be needed.

“At this point with a high level of protection in the UK population and no evidence of that being lost, to give third doses now in the UK whilst other countries have zero doses is not acceptable.”

Studies had previously shown that the shot, invented at Oxford University and licensed to AstraZeneca has higher efficacy when the second dose is delayed to 12 weeks instead of four weeks.

Monday’s research was released in a preprint, and looked at 30 participants who received a late second dose and 90 who received a third dose, all of whom were under 55.