Preliminary data suggests mixing COVID-19 vaccine increases reactogenicity

Researchers running the University of Oxford-led Com-COV study—launched earlier this year to investigate alternating doses of the Oxford-AstraZeneca vaccine and the Pfizer vaccine—have today reported preliminary data revealing more frequent mild to moderate reactions in mixed schedules compared to standard schedules.

Writing in a peer-reviewed Research Letter published in the Lancet, they report that, when given at a four-week interval, both of the ‘mixed’ schedules (Pfizer-BioNTech followed by Oxford-AstraZeneca, and Oxford-AstraZeneca followed by Pfizer-BioNTech) induced more frequent reactions following the 2nd, ‘boost’ dose than the standard, ‘non-mixed’ schedules. The add that any adverse reactions were short lived and there were no other safety concerns.

Matthew Snape, Associate Professor in Pediatrics and Vaccinology at the University of Oxford, and Chief Investigator on the trial, said:

‘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. 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.

‘Importantly, there are no safety concerns or signals, and this does not tell us if 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.’

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