Mixing doses of the Pfizer-BioNTech and AstraZeneca-Oxford vaccines generate a stronger immune response than having two rounds of the AstraZeneca shot, new U.K. research suggests, though the highest antibody response was seen in people being fully vaccinated with Pfizer.

The early findings, which haven’t yet been peer-reviewed, were released as a paper published online by the Lancet on Monday.

The results come from the University of Oxford-led Com-COV study, which is exploring the use of different combinations of approved COVID-19 vaccines.

This round of research looked at several vaccine combinations: two doses of Pfizer, two of AstraZeneca, and mixing doses with either AstraZeneca or Pfizer as the first shot and the other for the second.

“The mixed schedules did generate an immune response that was above the threshold set by the [AstraZeneca] vaccine, which we know is very effective against severe disease,” said the study’s chief investigator, Dr. Matthew Snape, an associate professor in pediatrics and vaccinology at the University of Oxford, during an interview with CBC News.

Both mixed schedules produced stronger responses than two doses of AstraZeneca, but the research team observed the highest antibody response in people receiving two doses of the Pfizer vaccine.

“Whether that translates into better protection or longer duration, we will have to see,” Snape said.

An AstraZeneca shot followed by Pfizer produced the best T-cell responses, the team found, and also a higher antibody response than Pfizer followed by AstraZeneca.

The results were for two doses of different vaccine combinations, given at four-week intervals to 830 people, in a participant-blind, non-inferiority trial — a type of study used to demonstrate that an experimental treatment is not substantially worse than an active treatment it’s being compared to.

Snape said he was surprised by the finding that the order of vaccines being used in the mixed schedules seemed to matter “quite a lot.”

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