Adults are more likely to report mild and moderate side-effects after mixing doses of the AstraZeneca and Pfizer Covid vaccines, a study indicates.
Chills, headaches, and muscle pain were reported more frequently when different vaccine doses were combined.
Any adverse reactions were short-lived, with no other safety concerns.
"It's a really intriguing finding and not something we were necessarily expecting," Prof Matthew Snape, from the Oxford Vaccine Group, said.
The Com-Cov study launched in February to see whether a different jab for the second dose might give longer-lasting immunity, better protection against new variants, or simply allow clinics to swap vaccines if supplies are interrupted.
The Canadian provinces of Ontario and Quebec have both said they plan to mix vaccines in the near future, amid uncertainty over shipments of the Oxford-AstraZeneca jab and concerns about rare blood clots.
The study, led by the University of Oxford, has recruited 830 volunteers aged over 50.
It is expected to publish its first full results in June.
But preliminary data has now been published in a research letter in the medical journal the Lancet.
Feverishness. Percentage reported after the second dose. Data showing the percentage of people who reported fatigue symptoms after the second dose.One in 10 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%
"The same real differences applied for other symptoms such as chills, fatigue, headache, malaise, and muscle ache," Prof Snape, the trial's chief investigator, said.
"One thing it's telling us is that you wouldn't want to vaccinate a ward full of nurses on the same day [with mixed doses of different vaccines], because you might have more absenteeism the next day."