Mixing Covid-19 vaccines tied to more side effects, early UK data suggests People who got mixed doses of coronavirus vaccines — receiving a different vaccine type as a second dose than the first dose — appear to be more likely to experience mild side effects such as fever, chills, fatigue or headache, researchers in the UK reported Wednesday. But the side effects following mix-and-match vaccinations were short-lived and there were no other safety concerns, the researchers reported in the Lancet medical journal. “These are the type of reactions you do expect with vaccine,” Dr. Matthew Snape, an associate professor of pediatrics and vaccinology at the University of Oxford and chief investigator on the trial, said during a media briefing.