And in the segments of the population that are most susceptible to the extreme effects of the flu -- like the elderly, who make up the majority of the cases of flu-related deaths -- the vaccinations are even less effective.

CNN: Flu vaccine attitudes abroad differ from U.S.

Ostroff says it may be because American doctors are counting milder cases, which appear more like a bad flu and less like pneumonia.

FORBES: Magazine Article

Although, in most cases, it causes little more than severe flu-like symptoms, occasionally there can be complications including inflammation of the lining of the heart.

BBC: Cull workers struck by rare fever