Why does influenza come back year after year?

After an epidemic, shouldn’t everyone be immune to the virus? Why is influenza able to come back again and again? The answer lies in the virus’s genetic structure and high mutation rate. The truth of the matter is that an individual’s immune system rarely sees the same influenza virus twice and that means it cannot provide the permanent protection it offers against more stable infectious agents.

Whenever you are infected by a pathogen, your immune system generates compounds called antibodies that bind to the infectious agent and target it for destruction. After the infection is cured, a few of the white blood cells that make those specific antibodies continue to circulate in your blood. Called “memory cells”, these cells are quickly activated  if the same infectious agent attacks you again, preventing the infectious agent from establishing itself and preventing you from getting sick. That is why vaccination works: a vaccine presents the immune system with a harmless form of an infectious agent so that if you are infected by the real thing, your immune system will be primed to destroy it quickly.

Unfortunately for humans, the influenza virus has evolved several strategies to outwit the immune system. Like all other organisms, the influenza virus changes every time it reproduces, generating progeny that are ever so slightly different from the parent.

Influenza viruses practice an extreme form of this lifestyle — they can change in several ways, some of them gradual and subtle, and some of them rapid and dramatic, but all of them allowing influenza to appear like a brand-new virus to the immune system. As a result, influenza strains change enough from year to year to allow them to repeatedly cause new epidemics.

The situation is made more complicated because there are at least four different influenza viruses circulating at any one time in the human population. The influenza virus family includes three main groups, called influenza A, B and C.

Only influenza A and B cause epidemics. Of these two types, there are generally two distinct strains in circulation, and the strains are different enough that immunity to one of them does not confer immunity to the others. The variety of influenza viruses in circulation guarantees that there will be “flu season” every year because last year’s flu victims will not have immunity if they encounter a different subtype or strain this year. It also greatly complicates the process of producing the annual influenza vaccine.

Influenza nomenclature
Influenza A strains are named according to which versions of two main proteins they carry on their surfaces, hemagglutinin and neuraminidase. So far, 16 varieties of hemagglutinin and 9 neuraminidases have been identified . So a strain called “H3N2” has hemagglutinin #3 and neuraminidase #2. Only strains carrying hemagglutinins #1 ,2, and 3 and neuraminidases 1 and 2 have caused epidemics in human. At present, both H3N2 and H1N1 viruses circulate widely in humans. All of the other varieties are found in wild birds, but some of those have caused serious zoonotics (outbreaks in an animal population) in domestic poultry.

“Source: This excerpt is taken from FAQ: Influenza, a report from the American Academy of Microbiology published in April 2013. The original, entire report can be downloaded free here, and this except is published with permission from American Academy of Microbiology.”