A Spanish study on mixing COVID-19 vaccines has found that giving a dose of Pfizer’s drug to people who already received the first shot of the 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 neutralizing 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 the first dose of AstraZeneca’s vaccine participated in the study, with some 450 given a Pfizer dose.
Just 1.7 percent 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.
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