Why it matters: The three- or four-week interval between the first and second doses of the Pfizer and Moderna vaccines was relatively short — and may help explain why the U.S. is now preparing for third doses.

What they’re saying: “When you make that decision to do a three- or four-week interval, it sacrifices length of protection and durability of protection,” said Peter Hotez, dean of the National School of Tropical Medicine at Baylor College.

He said it was a smart decision to pursue that vaccine interval early on, “because so many Americans were losing their lives from COVID — 3,000 deaths per day — and we had to get people fully immunized.”

But it also made boosters much more likely, he said.