This year's flu vaccine was less effective than officials previously thought, particularly for the elderly, said officials with the Centers for Disease Control and Prevention.
Officials have said the worst may be over and flu cases are declining. The 2012-13 flu season began in December, much earlier than usual, and was more severe than last year's.
That particularly was true for those 65 or older, according to a CDC report Thursday. The vaccine was 9 percent effective in protecting people in that age group.
"One possible explanation for this is that some older people did not mount an effective immune response to the H3N2 component of this season's vaccine," the report said. "However, it's not possible to say that for sure."
An H3N2 flu strain has been the most dominant and has been especially hard on the elderly.
Among those over 65, 146 per 100,000 were hospitalized for the flu virus, compared with a rate of 30 per 100,000 last year, the CDC said.
Dr. Thomas Frieden, the CDC's director, said this month in a congressional hearing that about twice as many elderly people have been hospitalized with flu symptoms than in recent years. That includes the 2007-08 and 2003-04 seasons, which also saw a predominant H3N2 strain.
"It seems that this year's flu vaccine is not nearly as effective as we would like among people over the age of 65 for one particular strain of flu," Frieden said Thursday. "That's why early treatment is so important."
Officials maintained, however, that those 65 and older should be vaccinated yearly, for reasons including that this population is at high risk of serious illness, hospitalization and death from the flu.
"This flu season has been worse than average and particularly severe for the elderly," Frieden said. "Although it's far from perfect, flu vaccination is by far the best tool we have to protect from the flu."
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."