In Texas alone, almost 6,000 people were overdue for their second shots in early February. Washington state officials said earlier this week that some mass vaccination clinics would only deliver follow-up doses. And a Michigan hospital system canceled last-dose appointments for the past week after its supply was reduced.

“All I have on hand is enough to get me through today and tomorrow,” Carolyn Wilson, chief operating officer of Beaumont Health system in Michigan, said earlier this week in an interview…

“For the first six weeks of this program or so, we were only giving first shots, and now we have to kind of pay the piper,” said Eric Toner, a senior scholar at the Johns Hopkins Center for Health Security.

Timing of the doses is seen as a key factor in effective immunization. Moderna Inc. says shots should be given four weeks apart to ensure efficacy, while the partnership of Pfizer Inc. and BioNTech SE recommends a three-week pause. A working group of Centers for Disease Control and Prevention advisers is considering recommendations for extending that interval, Bloomberg News has reported, although it still isn’t clear what effect that might have on protection.