Pfizer COVID-19 Vaccine to People Who Received First Dose of AstraZeneca Vaccine Safe and Effective, Finds Spanish Study

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New Delhi, May 18: A dose of Pfizer's COVID-19 vaccine to people who already received a first shot of Oxford-AstraZeneca's vaccine is "highly safe and effective", a study on mixing coronavirus vaccines has said. Conducted by Spain's state-backed Carlos III Health Institute, the Combivacs study 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 a first dose of Oxford-AstraZeneca's vaccine. Pfizer, Moderna COVID-19 Vaccines Effective Against Coronavirus Variants Found in India: Study.

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According to a report by Reuters, around 670 volunteers, who aged between 18-59 and had been vaccinated with a first dose of AstraZeneca's vaccine participated in the study. Of them, 450 participants were given Pfizer's COVID-19 vaccine as their second dose instead of AstraZeneca's. The study found that the presence of neutralising antibodies surged sevenfold after a Pfizer dose, significantly more than the doubling effect observed if a person receives AstraZeneca's second dose. What is the Moderna COVID-19 Vaccine? Does It Work, and is It Safe?

"These are not symptoms that can be considered serious," Dr Magdalena Campins, one of the study's leaders, was quoted as saying. Only 1.7 per cent of the participants showed severe side effects after mixing the vaccines. The side effects were limited to headaches, muscle pain and general malaise, she added. "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," Jesus Antonio Frias, clinical director at Carlos III, said.

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Earlier this month, results of a preliminary study published in the Lancet also showed that mixing COVID-19 vaccine doses - second dose from a different brand than the first - is safe, but may present more frequent mild side-effects such as a headache, chills or a fever.