By Thomas M. Burton WASHINGTON -- U.S. government scientists are pushing back against calls for one-dose regimens for two Covid-19 vaccines designed to be administered with two shots, saying there isn't enough evidence that a single dose provides long-term protection. "It is essential that these vaccines be used as authorized by FDA in order to prevent Covid-19 and related hospitalizations and death," Peter Marks, director of the Food and Drug Administration's center that oversees vaccines, told The Wall Street Journal. The FDA late last year approved a two-dose regimen for vaccines from Moderna Inc. and from a partnership of Pfizer Inc. and BioNTech SE. More recently it approved use of a one-dose regimen for a vaccine from Johnson & Johnson.