Mixing COVID-19 vaccine doses can increase chance of reactions, says study

The findings from the Oxford Vaccine Group's Com-Cov vaccine trial, which is studying the use of different combinations of approved COVID-19 vaccines for first and second doses were published Wednesday in a peer-reviewed medical journal as correspondence, not as a full study.

"Both of the mixed schedules caused more symptoms than the standard schedules," said study leader Dr. Matthew Snape, an associate professor in general pediatrics and vaccinology at the University of Oxford, during a media briefing on Wednesday before the publication of those early results.

"Most of these effects are mild," he said, though the findings did also show an increase in moderate reactions.

"Similar increases were observed for chills, fatigue, headache, joint pain, malaise, and muscle ache," the correspondence reads.

It also said it was "reassuring" the vaccine-induced symptoms were short-lived and that the limited data sparked no concerns.

More findings are awaited in the participant-blinded, randomized trial that has been taking place at a network of trial sites across the U.K.

This first round only shows the impact on post-vaccination reactions — not the overall safety or effectiveness of each mix-and-match approach.

"Whether or not this will relate to actually an improved immune response, we don't know yet," Snape said. "We'll be finding out those results in a few weeks' time."

Further studies, including those incorporating vaccines manufactured by Moderna and Novavax, are ongoing, the Lancet correspondence noted.

Ontario health officials also hope to explore the possibility of mixing two different COVID-19 vaccines.

Ontario's Health Minister Christine Elliott said Wednesday that the province is awaiting guidance from health regulators before taking any further decision after the use of AstraZeneca-Oxford supplies as a first dose was recently paused over blood clot concerns.

Currently the same two-dose schedule, with a first dose followed by a booster dose later, has been followed with all of the shots being used in Canada and the U.K., with the exception of the one-dose Johnson & Johnson shot, which has has not yet been administered in Canada, even though it has been approved for use in Canada.

(Reporting by Asha Bajaj)