Coronavirus: mixing Pfizer shot and AstraZeneca safe and effective, Spanish study finds

Around 670 volunteers who had already received a first dose of AstraZeneca’s shot took part in the Combivacs study

Just 1.7 per cent of the participants reported severe side effects, which were limited to headaches, muscle pain and general malaise

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A woman is administered the Pfizer Covid-19 vaccine in Pamplona, northern Spain. Photo: AP

Published: 9:28pm, 18 May 2021Updated: 9:28pm, 18 May 2021

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.

Meanwhile, the presence of neutralising antibodies rose sevenfold after a Pfizer dose, significantly more than the doubling effect observed after a second AstraZeneca shot.

Around 670 volunteers between the ages of 18-59 who had already received a first dose of AstraZeneca’s vaccine took part in the study, with some 450 given a Pfizer dose.

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.