The best treatment for influenza infections in humans is prevention by vaccination. Work by several laboratories has produced vaccines . The first H1N1 vaccine released in early October 2009 was a nasal spray vaccine that was approved for use in healthy individuals ages 2-49. However, the use of nasal spray has not been recommended since 2016. The injectable vaccine, made from killed H1N1, became available in the second week of Oct. 2009. This vaccine was approved for use in ages 6 months to the elderly, including pregnant females. Both of these vaccines were approved by the CDC only after they had conducted clinical trials to prove that the vaccines were safe and effective. A new influenza vaccine preparation is the intradermal (trivalent) vaccine is available; it works like the shot except the administration is less painful. It is approved for ages 18-64 years.
Influenza type A viruses undergo two major kinds of changes. One is a series of mutations that occurs over time and causes a gradual evolution of the virus. This is called antigenic "drift." The other kind of change is an abrupt change in the hemagglutinin and/or the neuraminidase proteins. This is called antigenic "shift." In this case, a new subtype of the virus suddenly emerges. Type A viruses undergo both kinds of changes; influenza type B viruses change only by the more gradual process of antigenic drift and therefore do not cause pandemics.