• Preliminary data from the so-called "Com-COV" study found that volunteers receiving alternating doses were more likely to develop mild to moderate symptoms.
• Some of those symptoms reported among participants receiving a mixed vaccine schedule included chills, fatigue, feverishness, headache, joint pain, malaise, muscle ache and pain at the injection site.
• The adverse reactions were found to be short-lived and there were no other safety concerns.

LONDON — A major U.K. trial assessing the benefits of mixing and matching Covid-19 vaccines has found that volunteers receiving alternating doses were more likely to develop mild to moderate symptoms.

The so-called "Com-COV" study, hailed as a "world-first" by the U.K. government and led by the University of Oxford, is examining the immune responses of trial participants given a dose of the Oxford-AstraZeneca vaccine followed by the Pfizer-BioNTech shot — and vice versa.

It reported on Wednesday that preliminary data shows participants receiving a mixed schedule of Covid-19 vaccines incurred more frequent reactions than those on standard non-mixed schedules.

Some of the mild to moderate symptoms reported among participants receiving a mixed vaccine schedule included chills, fatigue, feverishness, headache, joint pain, malaise, muscle ache and pain at the injection site.

The adverse reactions were found to be short-lived and there were no other safety concerns.

The data was recorded among participants aged 50 years and above. Researchers of the study said this means there is a possibility that adverse reactions to a mixed schedule of Covid-19 vaccines may be more prevalent in younger age groups.

It is thought that a mixed dosing schedule could lead to an increase in work absences the day after immunization against the coronavirus.

In a peer-reviewed research letter published in The Lancet international medical journal on Wednesday, researchers of the trial reported that when given at a four-week interval, both of the alternating vaccine schedules of the Oxford-AstraZeneca vaccine and the Pfizer-BioNTech vaccine induced more frequent reactions following the second dose than the standard non-mixed schedules.

"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," Matthew Snape, associate professor in Paediatrics and Vaccinology at the University of Oxford, said in a statement.

"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 immunisation of health care workers," he added.

'Com-COV2'

Snape, who is also chief investigator on the trial, said it was important to underline the fact that there were no safety concerns, adding it remained unclear whether the immune response will be affected.

"We hope to report these data in the coming months. In the meantime, we have adapted the ongoing study to assess whether early and regular use of paracetamol reduces the frequency of these reactions," Snape said.

The Com-COV study is backed by £7 million ($9.9 million) of U.K. government funding by the Vaccines Taskforce and run by the National Immunisation Schedule Evaluation Consortium.

The study aims to evaluate the feasibility of mixing and matching Covid vaccines to help policymakers understand whether this could be a viable route to increase the flexibility of vaccination campaigns.

The trial initially recruited 830 volunteers aged 50 and above. In April, researchers expanded the program to include the Moderna and Novavax Covid-19 vaccines in a new study, known as "Com-COV2." This added a further 1,050 volunteers to the program.