A Spanish study on mixing Covid-19 vaccines has found that giving a dose of Pfizer's drug to people who already received a first shot of AstraZeneca vaccine is highly safe and effective, preliminary results showed on Tuesday.

The Combivacs study, run by Spain's state-backed Carlos III Health Institute, found the presence of IgG antibodies in the bloodstream was between 30 and 40 times higher in people who got the follow-up Pfizer shot than in a control group who only received one AstraZeneca dose...