The flu season usually runs from November through March. Some years, cases continue into April and May. For the last 25 years, the heaviest flu activity has occurred in February.
Each year in the United States, on average, 5 to 20 percent of the population gets the flu. More than 250,000 people require hospitalization while about 36,000 die.
Influenza viruses can cause disease among people of any age, but rates of infection are highest among children. Serious illness and death are highest among persons ages 65 or older, and children ages 2 and younger. People of any age with certain medical conditions (e.g., congestive heart failure, asthma, diabetes), or who live in some type of long-term care facility are at risk for serious complications.
There are two types of influenza vaccine. The trivalent inactivated (killed) influenza vaccine, or TIV, may be used on any person age 6 months or older. This flu shot, an injection into muscle, usually is in the upper arm. The other is a live, attenuated (weakened) influenza vaccine, or LAIV, which is sprayed into the nostrils.
For most people, vaccination will prevent serious influenza-related illnesses. Vaccination will not prevent “flu-like” illnesses caused by other viruses. It takes two weeks for protection to develop after vaccination. So, vaccine that is given early in the flu season will provide a longer period of protection. Influenza vaccines can reduce infection in 70 to 90 percent of healthy adults under the age of 65.