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MADRID, May 18 (Reuters) - 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
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.
Spain embarked on the study to determine how best to proceed
after limiting AstraZeneca's shot to people aged over 60 due to
concerns about blood clotting in younger people.
That restriction caused widespread uncertainty and meant
some younger people who had already received a first dose have
been excluded from getting a second.
"Today's results support the possibility of vaccinating
patients who have received the first dose from AstraZeneca, but
the decision is not up to the investigators of this study," said
Jesus Antonio Frias, clinical director at Carlos III.
(Reporting by Nathan Allen, editing by Inti Landauro and Chizu
Nomiyama)
