May 18, 2021

By Jane Brown

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As hundreds of thousands of AstraZeneca COVID 19 vaccine doses sit unused in this country, researchers in Spain are reporting on a study that’s found giving a second dose of Pfizer vaccine to people who already received a first shot of AstraZeneca vaccine is highly safe and effective.

Scientists with Combivacs study, run by Spain’s state-backed Carlos III Health Institute, discovered IgG antibodies in the bloodstream were between 30 and 40 times higher in people who got the follow up Pfizer shot than people in a control group who only received one AstraZeneca dose.

Further, 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 a first dose of AstraZeneca’s vaccine participated in the study, with some 450 given a Pfizer dose.

Just 1.7% 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.

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.

We’re still waiting on word from Ontario’s public health experts on whether residents in this province who’ve had a first dose of AstraZeneca will be able to receive a second dose of the AZ vaccine.

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