US public health officials said a second Covid-19 shot could be administered as much as six weeks apart from the first one in situations where it was not possible to get a booster dose immediately.
In most cases, the recommended dosing interval of three weeks for Pfizer Inc's vaccine and four weeks for Moderna Inc's vaccine should be followed, the US Centers for Disease Control and Prevention (CDC) said in its guidance on Thursday.
However, "sometimes the situation is stressed where it's very difficult to be exactly on time", US infectious disease expert Dr. Anthony Fauci told CNN on Friday.
"I don't see a big problem with that (six weeks recommendation) if the situation on the ground means the stress is such that you can't precisely do 28 days or 21 days," Fauci added.
Moderna said there was no data to give an informed perspective on the CDC's recommendation, while Pfizer did not respond to Reuters' request for comment.
Britain has begun spacing doses of the vaccines by up to 12 weeks despite little data, saying the move could help more people get initial protection.