AstraZeneca COVID-19 vaccine (Photo: AFP/VNA)

The Combivacs study, run by Spain's state-backed Carlos III Health Institute, was conducted on 670 volunteers, aged 18-59, according to Reuters.

Dr Magdalena Campins, one of the study's leaders, said just 1.7% of the participants reported severe side effects, which were limited to headaches, muscle pain, and general malaise. "These are not symptoms that can be considered serious," she said.

The initial finding of a similar study in the UK was that people vaccinated with a dose of the Pfizer vaccine followed by a dose of AstraZeneca, or vice versa, were more likely to report mild 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.