Britain Opts for Mix-and-Match Vaccinations, Confounding Experts

Representatives of Public Health England and AstraZeneca did not respond to requests for comment.

Both Pfizer’s and AstraZeneca’s vaccines introduce into the body a protein called spike that, while not infectious itself, can teach immune cells to recognize and fight off the actual coronavirus.

But the vaccines impart their immunological lessons through different methods, and do not contain equivalent ingredients. While Pfizer’s vaccine relies on a molecule called messenger RNA, or mRNA, packaged into greasy bubbles, AstraZeneca’s shots are designed around a virus shell that delivers DNA, a cousin of mRNA.

Both vaccines are intended to be doled out in two-shot regimens, delivered three or four weeks apart. While the first shots of each vaccine are thought to be somewhat effective at preventing Covid-19, it’s the second dose — intended as a sort of molecular review session for the immune system — that clinches the protective process.

While it’s possible that swapping out one vaccine for another may still school the body to recognize the coronavirus, it is still a scientific gamble. With different ingredients in each vaccine, it’s possible people will not benefit as much from a second shot. Mixing and matching could also make it more difficult to collect clear data on vaccine safety.

Without evidence to back it, the hybrid vaccination approach seems “premature,” said Saad Omer, a vaccine expert at Yale University. Still, it’s not without precedent: Health authorities like the C.D.C. have previously said that if it’s impossible to give doses of a vaccine from the same manufacturer, “providers should administer the vaccine that they have available” to complete an injection schedule.

In a controversial move, the British government this week also decided to frontload its vaccine rollout, delivering as many first doses to people as possible — a move that could delay second shots up to 12 weeks.

The speedy deployment might afford more people partial protection against the virus in the short term. But some experts, including Dr. Moore, worry that this, too, might be unwise, and could imperil vulnerable populations.

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