Authorities earlier had said the flu vaccine was 62 percent effective for adults and children. In the Thursday report, the CDC revised that to 56 percent.
The new number, the agency said, is "not significantly different" and continues to fall within the confidence interval established earlier. But the lower number includes an additional three weeks of data and was adjusted for various factors, including age and race or ethnicity.
"Both estimates indicate moderate vaccine effectiveness in preventing outpatient medical visits due to circulating flu viruses in most of the population," the report said.
In addition, while this year's vaccine was considered a good match for the most common circulating flu viruses, it only provided 47 percent protection against H3N2, the main virus.
"We simply need a better vaccine against influenza, one that works better and lasts longer," Frieden said. The Department of Health and Human Services as well as the private sector are working on such a project, he said.
In addition, less than half of school-age children -- the population mainly responsible for spreading the flu virus -- got the flu shot, experts said. The CDC recommends the vaccine for those 6 months of age and older.
According to the latest numbers released Friday by the CDC, "influenza activity remained elevated in the United States, but decreased in most areas."