(Bloomberg) -- People may receive their follow-up doses of the Covid-19 vaccines as much as six weeks later if it’s not feasible to get them in the recommended interval, the Centers for Disease Control and Prevention said.
The guidance posted in a Jan. 21 update to the CDC website said a second dose should be administered as close to the recommended schedule as possible, either three weeks for the Pfizer Inc.-BioNTech SE vaccine or four weeks for the Moderna Inc. shot.
But if it’s impossible to get the follow-up shot on time, the CDC says people may schedule it as much as six weeks, or 42 days, after their initial dose. There is “limited data on efficacy” of the vaccines beyond that interval, according to the guidance, but if the second dose is administered later “there is no need to restart the series.”
Both vaccines authorized for emergency use in the U.S. were cleared based on trials that used two doses weeks apart. A grace period of four days ahead of schedule would be considered valid for a second dose, but people shouldn’t receive the second dose earlier than that.
The CDC also said the Covid-19 vaccines generally should be administered alone, not simultaneously with other inoculations.
The need for follow-up doses at specified intervals is one layer of complication in a logistically complex national vaccination campaign. Shortages of doses and confusion over supply has led to long waits and frustration, even as the incoming Biden administration pledges to accelerate the delivery of vaccines.
A single-dose Covid vaccine from Johnson & Johnson is in late-stage trials now, with data expected to be analyzed in the weeks ahead.