People who received different COVID-19 vaccines for their first and second dose were more likely to experience mild to moderate symptoms compared to those who received standard, non-mixed schedules, according to early findings led by researchers at the University of Oxford in the U.K.
The preliminary findings, published May 12 as a research letter in The Lancet,, compared symptoms between those who received the same vaccine for their first and second dose, to volunteers who received mixed schedules of Pfizer-BioNTech's shot followed by the Oxford-AstraZeneca vaccine, and vice versa.
Mild to moderate symptoms, including feverishness, chills and fatigue, were reported more frequently after the second dose among those who received mixed doses.
Adverse reactions were short lived and no other safety concerns were reported, researchers added.
"The results from this study suggest that mixed dose schedules could result in an increase in work absences the day after immunisation, and this is important to consider when planning immunization of healthcare workers," said Matthew Snape, MD, lead study investigator and associate professor in pediatrics and vaccinology at the University of Oxford.
Study participants were aged 50 and above, which suggests such reactions to mixed doses could be more common among younger age groups, according to researchers.
The preliminary findings are part of the U.K.'s larger "Com-COV" study, launched earlier this year to evaluate participants' immune response to mixed vaccine doses. The research on mixed doses being associated with more mild and moderate symptoms was an early, secondary finding.
Data on whether mixed dosing schedules affects the immune response will likely be reported in the coming months, researchers said.