Mixing Covid-19 vaccine increases reactogenicity

Adverse reactions were short-lived, with no other safety concerns.


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The Oxford Vaccine Group’s Com-Cov vaccine trial is contemplating the use of different combinations of approved COVID-19 vaccines for the first and second immunisation doses.

Launched earlier this year, the Com-COV study today reported preliminary information uncovering more successive mild to moderate responses in mixed schedules contrasted with standard schedules.

Scientists found that mixed dosing schedules of Pfizer / Oxford-AstraZeneca vaccines increase the frequency of mild-moderate symptoms in those receiving either mixed dosing schedule.

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. Any adverse reactions were short-lived, and there were no other safety concerns.

Matthew Snape, Associate Professor in Paediatrics and Vaccinology at the University of Oxford and Chief Investigator on trial, said“While 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 increase 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.”

Journal Reference:

  1. Robert H Shaw et al. Heterologous prime-boost COVID-19 vaccination: initial reactogenicity data. DOI: 10.1016/S0140-6736(21)01115-6


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