The COM-COV study, led by the University of Oxford, has been investigating the immune responses of volunteers given a dose of the Oxford/AstraZeneca vaccine followed by the Pfizer/BioNTech jab, and vice versa, since February. In its report for the medical journal 'Lancet' on Wednesday, the research team said adverse reactions from this mix and match approach were short-lived and there were no other safety concerns. The findings at this stage are limited to "reactogenicity" findings, or how people feel after the vaccine, and not the immunogenicity findings, that is how well the mixed dosing worked at inducing an immune response, work on which remains ongoing.

"The results from this study suggest 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," said Matthew Snape, associate professor in paediatrics and vaccinology at the University of Oxford.

''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," he said.

The mild side effects included symptoms like chills, fatigue, headaches and feeling feverish, and were short-lived. The research suggests 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.

One in 10 of over 800 volunteers given two AstraZeneca jabs four weeks apart reported feverishness – but if they received one AstraZeneca jab and one Pfizer, in any order, the proportion rose to about 34 per cent.

Last month, the study was expanded to add another 1,050 volunteers to test combinations of the Moderna and Novavax Covid vaccines, alongside the Pfizer and AstraZeneca, which is being produced in India as Covidshield.

The trial is not designed to show if the vaccines are effective at preventing disease and the University of Oxford has said the intent of the study is to show that mixing is not substantially worse than not mixing.

The Com-Cov study remit reads: "The purpose of this trial is to see how well people's immune systems respond when their second 'boost' dose is a different type of vaccine to their first "prime" dose.

"We will also be looking at how common vaccine reactions, such as fever, are after such 'mixed' schedules. This is important, as being able to use different vaccines in this way creates a more flexible immunisation programme; potentially allowing more people to be immunised more quickly." The researchers said they are enrolling people from all ethnicities and would particularly welcome participants from ethnic minority communities, considered among the higher-risk groups affected by Covid-19.