Influenza is a highly contagious respiratory illness caused by influenza viruses. The influenza vaccine has been shown to be effective in preventing illness in about 50-60% of healthy adults under the age of 65 years. This can vary by year, the person’s age and underlying medical conditions. It takes about 2 weeks for the vaccine to become effective and protect the individual against influenza. By vaccinating yourself against influenza you are not only protecting yourself, but everyone around you.