LONDON: A third shot of the Oxford-AstraZeneca COVID-19 vaccine produces a strong immune response, researchers said on Monday, adding there was not yet evidence that such shots were needed, especially given shortages in some countries.
The Oxford University study found that a third dose 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 enhanced immune response.
The British government has said it is looking at plans for an autumn vaccine booster campaign, with three-fifths of adults already having received both doses 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.
"We do have to be in a position where we could boost if it turned out that was necessary ... (but) we don't have any evidence that that is required," he told reporters.
"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.
It helps assuage concerns that viral vector COVID vaccines, such as those made by AstraZeneca and Johnson & Johnson, may lose their potency if annual inoculations are needed due to the risk that the body produces an immune response against the vectors that deliver the vaccine's genetic information.
"There had been some concerns that we would not be able to use this vaccine in a booster vaccination regime, and that's certainly not what the data is suggesting," study author Teresa Lambe of Oxford's Jenner Institute told Reuters.
Meanwhile, another Oxford study said on Monday that amixed schedule of vaccines where a shot of Pfizer’s COVID-19 vaccine is given four weeks after a AstraZeneca shot will produce better immune responses than giving another dose of the AstraZeneca shot.
The study, called Com-COV, compared mixed two-dose schedules of Pfizer and AstraZeneca vaccines, and found that in any combination, they produced high concentrations of antibodies against the coronavirus spike protein.
The data provides support for the decision of some European countries that have started offering alternatives to AstraZeneca as a second shot after the vaccine was linked to rare blood clots.
Matthew Snape, the Oxford professor behind the trial, said that the findings could be used to give flexibility to vaccine rollouts, but was not large enough to recommend a broader shift away from clinically approved schedules on its own.
“It’s certainly encouraging that these antibody and T-cell responses look good with the mixed schedules, but I think your default has to stay, unless there’s a very good reason otherwise, to what is proven to work,” he told reporters.
The highest antibody response was seen in people receiving two doses of Pfizer vaccine, with both mixed schedules producing better responses than two doses of AstraZeneca vaccine.
An AstraZeneca shot followed by Pfizer produced the best T-cell responses, and also a higher antibody response than Pfizer followed by AstraZeneca.
The results were for combinations of vaccines given at four week intervals to 830 participants.