With the summer dwindling down and Flu season about to start up, here's what you should know about this years flu season and shots.
There are four types of influenza viruses: A, B, C and D. Human influenza A and B viruses cause seasonal epidemics of disease (known as the flu season) almost every winter in the United States. Influenza A viruses are the only influenza viruses known to cause flu pandemics, i.e., global epidemics of flu disease.
A pandemic can occur when a new and very different influenza A virus emerges that both infects people and has the ability to spread efficiently between people. Influenza type C infections generally cause mild illness and are not thought to cause human flu epidemics. Influenza D viruses primarily affect cattle and are not known to infect or cause illness in people.
Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: hemagglutinin (H) and neuraminidase (N). There are 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes (H1 through H18 and N1 through N11, respectively). While there are potentially 198 different influenza A subtype combinations, only 131 subtypes have been detected in nature. Current subtypes of influenza A viruses that routinely circulate in people include: A(H1N1) and A(H3N2). Influenza A subtypes can be further broken down into different genetic “clades” and “sub-clades.” (See accompanying chart in pictures)
Of all the influenza viruses that routinely circulate and cause illness in people, influenza A(H3N2) viruses tend to change more rapidly, both genetically and antigenically. Influenza A(H3N2) viruses have formed many separate, genetically different clades in recent years that continue to co-circulate.
One influenza A(H1N1), one influenza A(H3N2), and one or two influenza B viruses (depending on the vaccine) are included in each season’s influenza vaccines. Getting a flu vaccine can protect against flu viruses that are like the viruses used to make vaccine. Information about this season’s vaccine can be found at Preventing Seasonal Flu with Vaccination. Seasonal flu vaccines do not protect against influenza C or D viruses. In addition, flu vaccines will NOT protect against infection and illness caused by other viruses that also can cause influenza-like symptoms. There are many other viruses besides influenza that can result in influenza-like illness (ILI) that spread during flu season.