Very few vaccines are 100% effective. Preliminary data for the 2010-2011 influenza season indicate that influenza vaccine effectiveness was about 60% for all age groups combined.
The flu vaccine is the single best way to prevent the flu, and vaccination is the main tool used to protect people from influenza. But, protection is never 100%, and some people can still get the flu after being vaccinated. How well the flu vaccine works (or its ability to prevent influenza illness) can range widely depending on who is being vaccinated. In general, the flu vaccine works best among young healthy adults and older children. Some older people and people with certain chronic illnesses might develop less immunity than healthy young adults after vaccination. However, even for these high-risk individuals, the flu vaccine still can provide protection against getting severe complications from the flu.
