Study suggests that AstraZeneca shot followed by Pfizer dose is safe and effective
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. 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.
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.
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.
Scientists speculate that, after the initial injection of the adenovirus-based vaccines, a person’s body develops an immune response to the adenovirus as well as the coronavirus spike protein. This immune response is problematic because it may mean that when the person receives the second dose of an adenovirus-based vaccine, the immune system targets and disables the adenovirus before it can deliver the instructions to the body’s cells to make the coronavirus spike protein, making the second jab less effective, according to Stephen Griffin, an associate professor of medicine at the University of Leeds who is not affiliated with the clinical trial. By using a different adenovirus for the second dose or a different vaccine technology, it is hoped this can be avoided.