Many health care workers and others at high risk who had the Pfizer shots in mid December lined up for their “booster” shot this week, due to be given 21 days after the initial dose.

The second shot carries many of the same logistical challenges as the first one, from cold chain requirements to administering the dose safely, plus the added challenge of ensuring the second dose is actually available to the correct people according to the drugmaker’s dosing timeline…

“The issue is really the unpredictable nature of the shipments,” says Maryellen Guinan of America’s Essential Hospitals, a group that represents more than 300 public hospitals around the U.S.

So far, there don’t appear to be widespread problems with people missing their second dose, but experts say it’s still early and the feasibility of a two-shot mass vaccine effort will be tested in the coming weeks, especially as those who received the Moderna vaccine await their follow up shot, which comes 28 days after the first.