( CNN ) -- 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 % 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 % effective for adults and children . In the Thursday report , the CDC revised that to 56 % . 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 % 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 . '' CNN 's Trisha Henry contributed to this report .
The flu vaccine showed 9 % effectiveness among those 65 and older , CDC says . The vaccine was about 56 % effective in adults and children , it says . New numbers show flu cases are declining in most areas .