The issue of mixing different vaccines has come up due to efforts to avoid supply shortages and changing recommendations concerning certain vaccines with reported severe side effects.
The study, published in the journal The Lancet, examined about 830 people who received two different vaccines, with 463 people receiving the second vaccine 28 days after the first one and 367 receiving the second vaccine 84 days after the first one.
The recently released data was gathered from self-reported symptoms collected seven days after both the first and second dose of the vaccine in participants who received the vaccines at 28-day intervals.
Two groups were included in the results: one that received the Oxford-AstraZeneca vaccine followed by the Pfizer-BioNTech vaccine and another that received them in the reverse order.
Both groups reported experiencing mild to moderate side effects at a greater rate than those who received the same vaccine both times. For example, feverishness was reported at nearly double the rate by those who received the Pfizer vaccine and then the AstraZeneca vaccine (41%) compared to those who received just the Pfizer vaccine for both doses (21%).
Participants reported similar increases for chills, fatigue, headache, joint pain, malaise and muscle ache, according to the study.
None of the participants were hospitalized for their side effects and most of the side effects were experienced within 48 hours of receiving the vaccine.
Paracetamol use also increased among those who received different vaccines for each dose compared to those who received the same vaccine for both doses.
The researchers noted that the participants were all 50-years-old or older and that younger groups may have higher rates of mild-to-moderate side effects after receiving two different vaccines.
Further studies incorporating the Moderna and Novavax vaccines are currently ongoing.
"Whilst this is a secondary part of what we are trying to explore through these studies, it is important that we inform people about these data, especially as these mixed-doses schedules are being considered in several countries," said Matthew Snape, associate professor in Pediatrics and Vaccinology at the University of Oxford and chief investigator on the trial, in a university press release."The results from this study suggest that mixed-dose schedules could result in an increase in work absences the day after immunization, and this is important to consider when planning immunization of health care workers," he said.
"Importantly, there are no safety concerns or signals, and this does not tell us if the immune response will be affected," Snape said.
The study's main focus is to understand how mixing vaccines affects immune response, with researchers stating that the results should be released in June.