In a report published Thursday in the New England Journal of Medicine, Australian researchers reported on a trial in 240 people ages 18 to 64, CNN said. Half the group received a single dose of 15 micrograms; half, 30 micrograms.
After 21 days, most of the subjects had an immune response in the range accepted by the U.S. Food and Drug Administration.
The National Institute of Allergy and Infectious Diseases reported that clinical trials in the United States have found the immune response is reduced in older people, so they may need additional doses.
Dr. Anthony Fauci, the agency's head, said in a statement that the tests also show that for healthy adults, only a single dose of vaccine may be necessary. He said adverse reactions to the vaccine are few.
Fauci and other experts had feared adults would need two doses, which would have put a strain on vaccine supplies.