EXPLAINER: US regulator weighs in on vaccine dosing debate

The first Americans vaccinated against COVID-19 are getting their second dose, while Britain has decided to postpone boosters and focus instead on giving more people a first shot — international differences that are adding to public confusion.

There's growing debate about whether to change vaccine dosing methods — the time between shots or even the amount in each shot — to stretch scarce supplies and possibly get more people inoculated faster. But the U.S. made clear late Monday that none of those strategies are on the table — because there's no science backing them.

“Making such changes that are not supported by adequate scientific evidence may ultimately be counterproductive to public health,” concluded a strongly worded statement from the Food and Drug Administration.

And despite all the attention to stretching supplies, the U.S. and other countries are facing logjams in using the doses that already have been raced out. Here are some questions and answers about vaccine dosing:

WHAT VACCINES ARE AVAILABLE?

That differs by country. The U.S. is allowing emergency use of two very similar vaccines, one made by Pfizer and Germany’s BioNTech and the other made by Moderna. Britain is using the Pfizer shot and a different type of vaccine made by AstraZeneca that also has been cleared in India. The European Union is rolling out the Pfizer shot.

Each requires two doses for full protection, either three weeks or four weeks apart depending on the vaccine.

COULD I WAIT LONGER BETWEEN DOSES?

Last week, British health officials decided it was OK to delay the second dose as long as 12 weeks. It was a huge surprise since none of the coronavirus vaccine studies were designed to test such a gap between doses.

British officials said postponing booster doses meant they...