Alternating the AstraZeneca and Pfizer vaccines causes frequent mild to moderate symptoms but safe

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Preliminary results of an ongoing study in the United Kingdom suggest alternating the Oxford-AstraZeneca and Pfizer-BioNTech vaccines causes more frequent mild to moderate symptoms, but there are no other safety concerns from mixing those vaccines.

However, researchers at the University of Oxford have not yet determined how a combination of shots would affect the immune system's response compared with sticking with the same COVID-19 vaccine for both the prime and booster shots.

In a peer-reviewed letter published in The Lancet, the scientists say that an increase in short-term adverse reactions occurred after the Pfizer vaccine was followed four weeks later by AstraZeneca, or vice versa, as part of the study that began earlier this year.

Results on whether immune response to mixed doses would be affected are expected to be released by the Oxford team in the coming months.

The study recruited 830 people to evaluate four combinations of vaccination: a first dose of AstraZeneca followed by either a booster shot of the Pfizer vaccine or another dose of AstraZeneca, or the Pfizer vaccine followed by a second shot of either AstraZeneca or Pfizer.

Research was expanded last month for a new study with 1,050 volunteers who received either the AstraZeneca or Pfizer vaccine before randomly getting either the same vaccine for their second dose or the Moderna or Novavax vaccine.

Horacio Bach, an infectious diseases expert at the University of British Columbia, said the small size of the initial study does not make it possible to know whether some people would get severe reactions, such as blood clots, from mixing the AstraZeneca and Pfizer vaccines.

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