Mixing Covid-19 vaccines increases the risk of unpleasant, short-term side-effects such as fatigue and fever, according to a UK trial in which people received different jabs for their first and second doses.

Preliminary results from the study run by Oxford university, published in The Lancet, showed a downside to vaccine mixing that the researchers had not expected, although the adverse reactions were shortlived. Mixed dosing is under consideration in several parts of the world, either because of safety concerns about the Oxford/AstraZeneca jab or to help spin out scarce supplies.

“The results from this study suggest that mixed dose schedules could result in an increase in work absences the day after immunisation,” said Matthew Snape, chief investigator on the trial, adding that it could be particularly important for planning the vaccination of healthcare workers.