Article by April M. Hopper, PharmD, Pharmacy Times
“Influenza is an acute respiratory illness caused by viruses that invade the nose, throat, and lungs. There are multiple types of influenza viruses, classified first by the antigenic properties of nucleoprotein and matrix protein antigens, as A, B, or C. Influenza A viruses are subtyped by surface hemagglutinin (H) and neuraminidase (N) antigen. Each A strain is further designated according to the site of origin, isolate number, year of isolation, and subtype. For example, this year’s influenza vaccine includes the A/Michigan/45/2015 (H1N1)pdm09-like virus as one of the A strains. B and C viruses are named in a similar manner but do not receive subtype classifications. There is not as much variation within the B viruses, and there may be no variation with the C viruses.
Outbreaks of influenza are recorded almost every year, with wide fluctuations in the extent and severity. The most severe outbreaks involve influenza A viruses, as the H and N antigens mutate frequently, by undergoing antigenic shifts or drifts. An antigenic shift is a major variation and may be associated with pandemics, while minor variations are known as drifts. These mutations make it essential for the composition of influenza vaccines to be reviewed each year and matched to the most commonly circulating viruses. ”