Researchers at Linkoping University studied how five different influenza epidemics struck in Ostergotland County of East Sweden from 2005 to 2010.
The study published in the journal PLoS ONE found the difference in those taken sick among age groups varies up to 10 times in certain cases with 2009 (H1N1) swine flu -- 2.3 cases per 1,000 inhabitants were diagnosed in those ages 10-19, compared with 0.2 cases in those age 70 and older age group before vaccinations were available.
In the flu outbreaks involving seasonal flu, the risk of falling ill was greatest in the 30-39 age group, and least for those age 70 and older.
The researchers pointed out that this data should be interpreted carefully, since many of the elderly are regularly vaccinated against seasonal influenza.
However, the study did not support the hypothesis that school children were a bigger risk group in general for influenza.
"One hypothesis is that the first influenza infection you get in your life affects your immune system," the researchers said in a statement. "This means that your immune system learns to react to one category of influenza, but has a poorer defense against other types. This is one of the theories we want to study."