“The second dose should be administered as close to the recommended interval as possible,” the CDC said in an update to its website Thursday. “However, if it is not feasible to adhere to the recommended interval, the second dose of Pfizer-BioNTech and Moderna Covid-19 vaccines may be scheduled for administration up to 6 weeks (42 days) after the first dose.”

Rina Shah, group vice president of pharmacy operations and services at Walgreens, which plans to administer doses at its stores across the country, said that “patients can receive the second dose as long as it follows the immunization window,” meaning that the second dose is given no earlier than the designated time frame.

“We encourage patients to schedule the second dose appointment as close to the earliest appropriate date, but a bit after is OK,” Shah said.

That runs counter to the official guidance from the Food and Drug Administration and Pfizer-BioNTech and Moderna, the drugmakers behind the two vaccines currently available in the U.S., which states that two doses, given at specific intervals, are necessary for maximum protection. For Pfizer, that interval is 21 days; for Moderna, 28 days.