Any adverse reactions - like headaches and chills - were short-lived though.
Oxford University experts have been investigating what happens when you have one shot of the Oxford jab, and another of the Pfizer one - compared to the standard dosing schedule.
Early data revealed that mixing doses increased in the frequency of mild to moderate symptoms - like fatigue or feeling fverish - compared to people given the same vaccine for both.
But, the researchers say there were no other safety concerns, and they don't yet know how well the mixed dosing worked at inducing an immune response.
Mixed-doses schedules are being considered in several countries.
The Com-Cov study was launched at Oxford in February to investigate alternating doses of the Oxford/AstraZeneca and Pfizer Covid-19 jabs.
Matthew Snape, associate professor in paediatrics 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 immunisation, and this is important to consider when planning immunisation of healthcare 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.”
Prof Snape added that the number of symptoms did not vary too much depending on which vaccine was given first.
He said: “They’re very very similar actually when you look at them.
“It slightly differs according to whether it was chills or fatigue that you’re looking at, but actually they were remarkably similar across the board.”
The researchers say that as the study data was recorded in participants aged 50 and above, there is a possibility such reactions may be more prevalent in younger age groups.