A contagious respiratory illness caused by influenza viruses. Annual vaccination is the best way to prevent illness and serious complications.
Influenza (flu) is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and sometimes the lungs. It can cause mild to severe illness, and at times can lead to death. The best way to prevent flu is by getting a flu vaccine each year (CDC, 2024).
Flu seasons vary in timing, severity, and length. In the United States, flu activity typically peaks between December and February, but can last as late as May. The CDC estimates that flu has resulted in 9-41 million illnesses, 140,000-710,000 hospitalizations, and 12,000-52,000 deaths annually since 2010 (CDC, 2024).
Influenza viruses are constantly changing (antigenic drift), which is why new flu vaccines are developed each year. The WHO Global Influenza Surveillance and Response System (GISRS) monitors influenza viruses globally and recommends which strains should be included in flu vaccines for the upcoming season.
There are four types of influenza viruses: A, B, C, and D. Types A and B cause seasonal epidemics in humans. Influenza A viruses are further classified into subtypes based on surface proteins hemagglutinin (HA) and neuraminidase (NA), such as H1N1 and H3N2.
Vaccine effectiveness varies each year depending on how well the vaccine strains match circulating strains, the health and age of the recipient, and other factors. Effectiveness typically ranges from 30-60%.