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New coronavirus vaccine puzzle: who should get which shot?

First came the good news. The European Union authorized a third vaccine.

Then, the bad news. Regulators in country after country suggested restricting it to younger people until more testing was done.

The decisions marked the start of a delicate new phase of vaccination drives, one in which a growing menu of coronavirus vaccines was accompanied by contentious debates about who should be given which shots.

Those debates are a testament to the world’s good fortune in having several strong vaccines only a year into the pandemic. But every vaccine comes with its own idiosyncrasies, including gaps in clinical trial data. And that has thrown up agonizing choices for countries already struggling to administer shots, forcing health officials to weigh their qualms about certain vaccines with the need to inoculate people before dangerous variants take hold.

After the European Union authorized the AstraZeneca-Oxford vaccine last week, adding a third shot to the bloc’s arsenal, Germany, Italy, France, Belgium, Lithuania, Poland, Austria and Sweden all said they would restrict it to younger people or were considering doing so, citing a scarcity of data on the vaccine’s efficacy in older people.

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Under those plans, older people would instead be scheduled to receive the Pfizer-BioNTech or Moderna vaccines, potentially leaving them unvaccinated for a period even as younger people are inoculated with the AstraZeneca shot.

But those strategies, tangled as they may be, will at least make a new vaccine available to younger people, scientists said, a scenario far preferable to no one receiving AstraZeneca’s shot.

Some scientists are now urging the United States to adopt the same approach, as the country remains without an alternative to the hard-to-store Pfizer and Moderna vaccines. Its regulators refuse to authorize AstraZeneca’s shot until another clinical trial generates more data, including on how it works in older people.

In the absence of that data, targeting the vaccine to those in whom it is known to be effective was an urgently needed stopgap, scientists said — all the more so now that the virus is rapidly acquiring new and dangerous mutations.

“This is a pragmatic solution to a desperate situation,” said Dr. Peter Hotez, a vaccine scientist at the National School of Tropical Medicine at Baylor College of Medicine in Texas. “Everything’s changed. The whole Biden plan made perfect sense up until about three to four weeks ago when we realized the pace of the variants emerging, and therefore you have to adjust.”

Confusion bubbled up in Italy after regulators there said AstraZeneca’s vaccine should, with some exceptions, “preferably” be used on adults under 55. Pfizer and Moderna’s shots, both mRNA vaccines that have shown roughly 95% efficacy in preventing COVID-19, should be given to the most vulnerable people, they said.

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