COVID-19 boosters are coming but who will get them and when?

COVID-19 booster shots may be coming for at least some Americans but already the Biden administration is being forced to scale back expectations — illustrating just how much important science still has to be worked out.

The initial plan was to offer Pfizer or Moderna boosters starting Sept. 20, contingent on authorization from U.S. regulators. But now administration officials acknowledge Moderna boosters probably won't be ready by then — the Food and Drug Administration needs more evidence to judge them. Adding to the complexity, Moderna wants its booster to be half the dose of the original shots.

As for Pfizer's booster, who really needs another dose right away isn't a simple decision either. What's ultimately recommended for an 80-year-old vaccinated back in December may be different than for a 35-year-old immunized in the spring — who likely would get a stronger immunity boost by waiting longer for another shot.

FDA's scientific advisers will publicly debate Pfizer's evidence on Sept. 17, just three days before the administration's target. If the FDA approves another dose, then advisers to the Centers for Disease Control and Prevention will recommend who should get one.

That's tricky because while real-world data shows the vaccines used in the U.S. remain strongly protective against severe disease and death, their ability to prevent milder infection is dropping. It's not clear how much of that is due to immunity waning or the extra-contagious delta variant — or the fact that delta struck just as much of the country dropped masks and other precautions.

When to jump to boosters "becomes a judgment,” said Dr. Jesse Goodman of Georgetown University, a former FDA vaccine chief. "And is that urgent or do we have time for the data to come in?”

Already the CDC is...