A mixed schedule of COVID-19 vaccines where a dose of Pfizer is given four weeks after an AstraZeneca dose will produce better immune responses than giving another dose of AstraZeneca, say UK researchers.

The pre-print 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 University researcher 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.