Vaccination is the most effective way to prevent influenza. Effectiveness of the vaccination varies with the age and immune status of the vaccinated person, the (sub)type of the virus, the degree of similarity between the vaccine and the circulating virus strains, and the length of time from vaccination to exposure to influenza virus. It is about 70–90% effective in healthy people under 65 years old and usually lower among the elderly and those with chronic medical conditions. Among the high-risk groups, influenza vaccination has been shown to be most effective in preventing severe illness, secondary complications and death. In addition to the 70-90% rate of efficacy, vaccines achieve average effectiveness of 25%, i.e. they are capable of reducing the number of cases of influenza-like illness by 25%.