Pfizer jab after first AstraZeneca dose safe and highly effective: Study

Just 1.7 per cent of the participants reported severe side effects, which were limited to headaches, muscle pain and general malaise, said Dr Magdalena Campins, one of the study's leaders.

"These are not symptoms that can be considered serious," she said.

Spain undertook the study to determine how best to proceed after limiting AstraZeneca's shot to people aged over 60 due to concerns about blood clotting in younger people.

In a U.K. "mix-and-match" study, first findings recently showed that people vaccinated with a shot of Pfizer's vaccine followed by a dose of AstraZeneca's, or vice versa, were more likely to report mild or moderate symptoms such as headaches or chills than if they received two of the same type. Data on immune responses are expected in the coming months.

That restriction caused widespread uncertainty and meant some younger people who had already received a first dose have been excluded from getting a second.

"Today's results support the possibility of vaccinating patients who have received the first dose from AstraZeneca, but the decision is not up to the investigators of this study," said Jesus Antonio Frias, clinical director at Carlos III.