Spain says mixing AstraZeneca and Pfizer is safe and effective, limited side effects noted

A Spanish study has suggested that administering a second dose of Pfizer to those who received AstraZeneca as their first dose is safe. It saw antibodies increase seven-fold compared to those who were given no second jab at all.

The CombivacS study, which enrolled 676 participants who had already received a first dose of AstraZeneca and gave them a second dose of Pfizer, has produced positive preliminary results. Of the total cohort, 450 people were given Pfizer as their second dose, and the remaining 226 have not received any second dose and are being observed as the control group. 

Published on Tuesday, the study, launched by the Carlos III Health Institute, reported that the combination was both safe and effective. Jesus Antonio Frias, coordinator of the ISCIII Clinical Research Network, said, “A more than seven-fold increase in neutralizing antibodies was observed 14 days after vaccination.”  

According to Magdalena Campins, of the Vall d’Hebron University Hospital in Barcelona, very few side effects have been reported, other than minor pain at the injection site and general discomfort or headaches.

The data reinforces the findings of a larger British study that published its preliminary results last week. It said there were no immediate safety concerns about mixing AstraZeneca and Pfizer jabs, but noted that adverse reactions were heightened. The researchers suggested it was possible that these were more prevalent among younger people, but did not elaborate. 

Spain commenced its study following the government’s decision to stop using AstraZeneca for those under the age of 60. The study’s findings will inform whether the nearly two million essential workers who received a first dose of AstraZeneca’s Covid-19 shot will receive a second dose, and which jab they will be given.

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