Influenza, also known as the flu, is a contagious disease that is caused by the influenza virus. It attacks the respiratory tract (nose, throat, and lungs) in humans. Influenza usually occurs during the months of November through April.
Influenza types A or B viruses cause epidemics of disease almost every winter. In the United States, these winter influenza epidemics can cause illness in 10% to 20% of people and are associated with an average of 36,000 deaths and 114,000 hospitalizations per year
Flu seasons are unpredictable in a number of ways. Although epidemics of flu happen every year, the timing, severity, and length of the epidemic depends on many factors, including what influenza viruses are spreading and whether they match the viruses in the vaccine.
Influenza is a serious disease that can lead to hospitalization and sometimes even death. Every flu season is different, and influenza infection can affect people differently. Even healthy people can get very sick from the flu and spread it to others. Studies going back to 1976 have found that flu-related deaths ranged from a low of 4,700 to a high of 56,600 (average 25,500). During a regular flu season, about 90 percent of deaths occur in people 65 years and older. The “seasonal flu season” in the United States is usually from November through April each year.
During this time, flu viruses are circulating in the population. An annual seasonal flu vaccine (either the flu shot or the nasal-spray flu vaccine) is the best way to reduce the chances that you will get seasonal flu and lessen the chance that you will spread it to others. When more people get vaccinated against the flu, less flu can spread through that community.
CDC recommends that people get their seasonal flu vaccine as soon as vaccine becomes available in their community. Vaccination before December is best since this timing ensures that protective antibodies are in place before flu activity is typically at its highest. CDC continues to encourage people to get vaccinated throughout the flu season, which can begin as early as October and last as late as May. Over the course of the flu season, many different influenza viruses can circulate at different times and in different places. As long as flu viruses are still spreading in the community, vaccination can provide protective benefit.
FDA has licensed two forms of influenza vaccine for use in the United States: the inactivated vaccine (sometimes called the "flu shot") and the live attenuated vaccine, which is a nasal spray.
The inactivated vaccine contains inactivated, or killed, virus and is given with a needle in the arm. The nasal spray vaccine contains live viruses that are weakened, or attenuated, and is administered into the nose with a nasal sprayer.
Neither vaccine will cause influenza.
The Centers for Disease Control and Prevention’s (CDC) Advisory Committee on Immunization Practices (ACIP) provides annual recommendations for the prevention and control of influenza, including use of vaccines. As of this year, they recommend that everyone in the United States six months of age and older receive the seasonal influenza vaccine.
Your body’s level of immunity from a vaccine received last season is expected to have declined. You may not have enough immunity to be protected from getting sick this season. You should be vaccinated again to raise your immune levels against the three viruses that research indicates are likely to circulate again this season.