Mixing Pfizer and AstraZeneca vaccines supplies robust safety, in accordance with a preliminary research.

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Mixing Pfizer and AstraZeneca vaccines provides strong protection, according to a preliminary study.

Initial results from a UK vaccine study suggest that mixing different brands of vaccine can produce a protective immune response against Covid-19. In the study, volunteers produced high levels of antibodies and immune cells after receiving a dose of the Pfizer BioNTech vaccine and a dose of the AstraZeneca Oxford shot.

Giving the vaccines in any order is likely to provide effective protection, Matthew Snape, a vaccines expert at Oxford University, told a news conference Monday. “Any of these schedules I think could be argued and expected to be effective,” he said.

Dr. Snape and his colleagues started the study called Com-COV in February. In the first wave of the study, they gave 830 volunteers one of four vaccine combinations. Some received two doses from Pfizer or AstraZeneca, both of which have been shown to be effective against Covid-19. Others got a dose of AstraZeneca, followed by one from Pfizer, or vice versa.

With the first wave of volunteers, the researchers waited four weeks between doses. Studies have shown that the AstraZeneca vaccine offers greater protection when the second dose is delayed for up to 12 weeks. Therefore, the researchers are also conducting a separate 12-week study that should provide results over the next month.

The researchers found that volunteers reported more chills, headaches, and muscle aches than people who received two doses of the same vaccine. But the side effects were short-lived.

Dr. Snape and his colleagues then took blood samples to measure the immune response in the volunteers. They found that those who received two doses of Pfizer-BioNTech produced antibody levels about 10 times higher than those who received two doses of AstraZeneca. Volunteers who received Pfizer followed by AstraZeneca showed antibody levels about five times higher than those who received two doses of AstraZeneca. And volunteers who received AstraZeneca followed by Pfizer achieved antibody levels about as high as those who received two doses of Pfizer.

Dr. Snape said the differences would most likely narrow in the volunteers who received a second dose after 12 weeks when the AstraZeneca vaccine had more time to intensify its effects.

The study also found that using different vaccines produced higher levels of immune cells prepared to attack the coronavirus than giving two doses of the same vaccine. Dr. Snape said it was not yet clear why mixing had this advantage. “It’s very fascinating, let’s say so much,” he said.

Dr. Snape and colleagues have started a similar study, adding Moderna and Novavax vaccines to their list of possibilities.

For now, he said, the best course of action remains to get two doses of the same vaccine. Large clinical studies have clearly shown that this strategy reduces the likelihood of contracting Covid-19. “Your default should be what has been shown to work,” said Dr. Snape.

But there are many cases where that is not possible. Vaccine deliveries are sometimes delayed due to manufacturing issues, for example. In some countries, younger people have been advised not to receive a second dose of AstraZeneca due to concerns about the low risk of blood clots forming. In situations like this, it’s important to know if people can switch to another vaccine.

“This provides reassuring evidence that should work,” said Dr. Snape.

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