In the United States, influenza, commonly known as the flu, typically occurs between January and February. However, seasonal flu activity can last from fall through spring, spanning the months of October through May. Most Americans know of the flu as a common illness, hardly a rare disease. Despite its annual occurrence, the development of a new flu strain is always unpredictable.
Why do influenza viruses mutate?The influenza virus is a virus that undergoes an incredible amount of change every year. A new flu strain can develop in the following ways:
- Antigenic drift refers to the small, gradual changes that occur to viruses, causing new flu strains to be created. As increasing amounts of new flu strains appear, antibodies can no longer recognize the older virus, causing reinfection to occur. This is the main reason that people get the flu multiple times throughout their lives, or even during one standard flu season. Both influenza type A and type B viruses can undergo antigenic drift.
- Antigenic shift refers to abrupt, substantial change in influenza A viruses, resulting in new proteins in influenza viruses that infect humans. The shifts tend to migrate from an animal population to a human population, meaning that humans generally don't have any immunity to this novel virus. Unlike antigenic shift, antigenic drift occurs only occasionally, such as with the H1N1 virus of 2009.
How can we prevent new flu strains from spreading?According to the Centers for Disease Control (CDC), three kinds of flu viruses circulate in the United States:
- Influenza A (H1N1) viruses
- influenza A (H3N2) viruses
- Influenza B viruses
- an A/California/7/2009 (H1N1)pdm09-like virus
- an A(H3N2) virus antigenically like the cell-propagated prototype virus A/Victoria/361/2011
- a B/Massachusetts/2/2012-like virus