ℙ𝕗𝕚𝕫𝕖𝕣, 𝕄𝕠𝕕𝕖𝕣𝕟𝕒 𝕧𝕒𝕔𝕔𝕚𝕟𝕖𝕤 𝕔𝕠𝕦𝕝𝕕 𝕡𝕣𝕠𝕥𝕖𝕔𝕥 𝕒𝕘𝕒𝕚𝕟𝕤𝕥 ℂ𝕆𝕍𝕀𝔻-𝟙𝟡 𝕗𝕠𝕣 𝕪𝕖𝕒𝕣𝕤: 𝕤𝕥𝕦𝕕𝕪

𝕋𝕙𝕖 𝕧𝕒𝕔𝕔𝕚𝕟𝕖𝕤 𝕞𝕒𝕕𝕖 𝕓𝕪 ℙ𝕗𝕚𝕫𝕖𝕣-𝔹𝕚𝕠ℕ𝕋𝕖𝕔𝕙 𝕒𝕟𝕕 𝕄𝕠𝕕𝕖𝕣𝕟𝕒 𝕤𝕖𝕥 𝕠𝕗𝕗 𝕒 𝕡𝕖𝕣𝕤𝕚𝕤𝕥𝕖𝕟𝕥 𝕚𝕞𝕞𝕦𝕟𝕖 𝕣𝕖𝕒𝕔𝕥𝕚𝕠𝕟 𝕚𝕟 𝕥𝕙𝕖 𝕓𝕠𝕕𝕪 𝕥𝕙𝕒𝕥 𝕞𝕒𝕪 𝕡𝕣𝕠𝕥𝕖𝕔𝕥 𝕒𝕘𝕒𝕚𝕟𝕤𝕥 𝕥𝕙𝕖 𝕔𝕠𝕣𝕠𝕟𝕒𝕧𝕚𝕣𝕦𝕤 𝕗𝕠𝕣 𝕪𝕖𝕒𝕣𝕤, 𝕤𝕔𝕚𝕖𝕟𝕥𝕚𝕤𝕥𝕤 𝕙𝕒𝕧𝕖 𝕣𝕖𝕡𝕠𝕣𝕥𝕖𝕕.

The findings add to growing evidence that most people immunised with the mRNA vaccines may not need boosters, so long as the virus and its variants do not evolve much beyond their current forms – which is not guaranteed. People who recovered from COVID-19 before being vaccinated may not need boosters even if the virus does make a significant transformation.

“It’s a good sign for how durable our immunity is from this vaccine,” said Ali Ellebedy, an immunologist at Washington University in St Louis who led the study, which was published in the journal Nature on Monday, local time.

Ellebedy and his colleagues reported last month that in people who survived COVID-19, immune cells that recognise the virus lie quiescent in the bone marrow for at least eight months after infection. A study by another team indicated that so-called memory B cells continue to mature and strengthen for at least a year after infection.

Based on those findings, researchers suggested that immunity might last for years, possibly a lifetime, in people who were infected with the coronavirus and later vaccinated. But it was unclear whether vaccination alone might have a similarly long-lasting effect.

Ellebedy and his colleagues recruited 41 people – including eight with a history of infection with the virus – who were immunised with two doses of the Pfizer-BioNTech vaccine.

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Ellebedy’s team found that 15 weeks after the first dose of vaccine, a specialised structure called the germinal centre that forms in lymph nodes was still highly active in all 14 of the participants, and that the number of memory cells that recognised the coronavirus had not declined.

The results suggest that a vast majority of vaccinated people will be protected over the long term – at least, against the existing coronavirus variants. But older adults, people with weak immune systems and those who take drugs that suppress immunity may need boosters; people who survived COVID-19 and were later immunised may never need them at all.

Meantime, early results from a British vaccine study suggest that mixing different brands of vaccines can provoke a protective immune response against COVID-19. In the trial, volunteers produced high levels of antibodies and immune cells after getting one dose of the Pfizer-BioNTech vaccine and one dose of the AstraZeneca-Oxford shot.

Administering the vaccines in either order is likely to provide potent protection, Dr Matthew Snape, a vaccine expert at the University of Oxford, said at a news conference on Monday, local time. “Any of these schedules, I think could be argued, would be expected to be effective,” he said.

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

For the first wave of volunteers, the researchers waited four weeks between doses. Studies have found that the AstraZeneca vaccine provides stronger protection if the second dose is delayed for up to 12 weeks, so the researchers are also running a separate 12-week trial which should deliver results next month.

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

Dr Snape and his colleagues then drew blood to measure the immune response in the volunteers. They found that those who got two doses of Pfizer-BioNTech produced levels of antibodies about 10 times as high as those who got two doses of AstraZeneca. Volunteers who got Pfizer followed by AstraZeneca showed antibody levels about five times as high as those with two doses of AstraZeneca. And volunteers who got AstraZeneca followed by Pfizer reached antibody levels about as high as those who got two doses of Pfizer.

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

The study also found that using different vaccines produced a higher level of immune cells primed to attack the coronavirus than did giving two doses of the same vaccine. Dr Snape said it wasn’t clear yet why mixing had that advantage. “It’s very intriguing, let’s say that much,” he said.

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Dr Snape and his colleagues have begun a similar trial, adding vaccines from Moderna and Novavax to the list of possibilities.

For now, he said, the best course of action remains getting two doses of the same vaccine. Large clinical trials have clearly demonstrated that this strategy reduces the chances of getting COVID-19. “Your default should be what is proven to work,” Dr. Snape said.

But there are many cases in which that may not be possible. Vaccine shipments are sometimes delayed because of manufacturing problems, for example. Younger people in some countries have been advised not to get a second dose of AstraZeneca because of concerns about the small risk of developing blood clots. In such situations, it’s important to know whether people can switch to another vaccine.

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

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