MADRID -- 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, the researchers said on Tuesday.

The so-called Combivacs study, run by Spain's state-backed Carlos III Health Institute, found the immune response in people who received a Pfizer shot was between 30 and 40 times greater than in a control group who only had AstraZeneca dose.

Few serious side effects were reported among the 600 participants, the authors said.