This season's flu hospitalization rates in those 65-plus is the highest since CDC began its current surveillance system in 2007, said Michael Jhung, a CDC epidemiologist. In the last week of January, the rate of people in that age group who were hospitalized with a laboratory-confirmed case of influenza was 116 per 100,000. Previously, the highest rate was 73.7 per 100,000, he said.
The CDC findings are consistent with studies of how well this season's flu virus worked in Europe. The CDC researchers cautioned that the findings were interim and looked only at people who had gone to the doctor with flu symptoms. More research is needed to see if chronic medical conditions and other problems associated with aging might have affected the outcome, they said. The CDC plans to do further research.
Overall, the vaccine's effectiveness for everyone older than 6 months was 56%, just slightly lower than the 62% that had been estimated earlier in the season. This season's vaccine contains protection against three flu strains: H3N2, influenza B and H1N1. The vaccine was 67% effective against influenza B in adults over 65 but only 9% effective against H3N2, the most prevalent strain this season, the CDC found. There were not enough H1N1 to tell its effectiveness.
When broken into age groups, the vaccine's overall effectiveness against H3N2 flu was:
• 6 months to 17 years: 58%
• 18 to 49 years: 46%
• 50 to 64 years: 50%
• 65 and older, 9%.
As people age, the immune system becomes less able to battle sickness. Some studies in past flu seasons have found the vaccine to be a strong benefit for older adults, some less. A study that looked at several years worth of data found the vaccine reduced the risk of influenza-associated hospitalizations among older adults by 61%.