In healthy, young adults the flu vaccine is up to 90 per cent effective in preventing the flu. For those who get the flu despite being vaccinated, the illness is usually much less severe. In the elderly, the vaccine is less effective in preventing flu but is very effective in preventing hospitalization and death.

Possible explanations for getting flu-like illness despite being vaccinated are:

• The vaccine didn't give full protection and the person got true influenza. This will happen to a few people, but without vaccination the illness would probably be much more severe.
• Many other illnesses have flu-like symptoms. Flu vaccine doesn't prevent these other illnesses. It only protects you against true influenza. The flu-like symptoms are probably due to infection by another virus.