LONDON (REUTERS) – A mixed 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, an Oxford study said on Monday (June 28).

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.

Professor Matthew Snape, the Oxford don behind the trial, said that the findings could be used to give flexibility to vaccine roll-outs, 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.

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