Influenza is an illness caused by RNA viruses that infect the respiratory tract of many animals, birds, and humans. Compared with most other viral respiratory infections, such as the common cold, influenza infection can cause a more severe illness or even death. Influenza viruses are divided into three types, designated A, B, and C.
Influenza types A and B, which are responsible for respiratory illness, occur almost every winter and are often associated with increased rates of hospitalization and death. Influenza type C differs from types A and B in that it causes either a very mild respiratory illness or is asymptomatic, therefore a severe public-health impact is not created.
Influenza A viruses are negative sense, single-stranded, segmented RNA viruses with several subtypes. Human influenza virus usually refers to subtypes which spread widely among humans with the following subtypes currently circulating in the USA ; 2009 H1N1 and H3N2. The Influenza type A virus undergoes two transformations, one a series of mutations occurring over time resulting in a gradual evolution of the virus, while the other is a more abrupt change of the hemagglutinin (H) or neuraminidase (N) proteins, resulting in the emergence of a new virus subtype. However, influenza type B viruses change gradually through antigenic drift thereby preventing pandemics. Currently the Victoria and Yamagat Lineages are prevalent. The severity of the infection will depend in large part on the state of the infected person’s immune system and if the victim has previously been exposed to the current strain, which would result in partial immunity.
Vaccination is the most effective way to prevent infection caused by influenza viruses. For more than 50 years, WHO has been collaborating with scientists and policy makers on a global scale to develop a unified approach to manufacturing, testing and regulating influenza vaccine development. Although safe and effective, vaccines have been available and used for more than 60 years. Among healthy adults, influenza vaccines can prevent 70% to 90% of influenza-specific illness while reducing severe illness and complications in the elderly by up to 60%, and deaths by 80%.
