Many vaccination clinics and local officials are reporting long lines and delays in booking vaccination appointments recently, the product of expanded eligibility on booster shots and fears of the Omicron variant, experts said.

The stresses on the U.S. vaccination program are also worsened by the broader labor shortage that is affecting many sectors, including health care.

“What the public needs to keep in mind is that we’ve got a stressed and strained health care delivery system,” said Mitchel Rothholz, the lead for immunization policy at the American Pharmacists Association.

Vaccine demand has spiked from an average of under a million doses a day for much of October to an average of 1.5 million a day in recent weeks, according to data from the Centers for Disease Control and Prevention. Demand for boosters and first time doses seems to be driving that increase.