The Influenza Virus: Why do we keep getting it?

Viruses are very tiny particles full of proteins and genetic material. And they have a job to do: infect our bodies/cells and make lots and lots of copies of themselves. The influenza virus in particular targets our lungs, nose, and throat. Then, it invades our cells, sets up shop, and goes to town making millions of copies of itself.

Soon millions of viral clones are attacking the body, leaving you feeling achy, tired, and just plain sick. But, after a while our bodies figure out they are under attack and the immune system runs to the rescue. Antibodies are created and the virus is banished, leaving the immune system with the memory to be able to attack it should it ever enter the body again.

But, the influenza virus is different from a lot of viruses in that it mutates and changes over the years, making it unrecognizable to our immune systems. Therefore, the virus can infect your body many times and leave you feeling sick over and over and over again.

So what’s going on here that’s different from other viruses?

First, here’s what that nasty influenza virus particle looks like (above).

Those little things jetting out from the particle are called glycoproteins. These little glycoproteins are vital in understanding how the virus works. There are two different kinds of glycoproteins on the surface of the influenza virus and those are: haemagglutinin (HA) and neuraminidase (NA). HA is the major glycoprotein and is about four times more abundant than NA.

There are at least 16 different subtypes of HA, noted as H1 through H18. And there are at least 9 different subtypes of NA, noted N1 through N9. The two subtypes of HA and NA give us the subtype name for the virus.

For example, here are some subtypes you may have heard of most recently:

• H1N1 caused the swine flu outbreak in 2009
• H5N1 caused the avian flu in 2004

Furthermore, the influenza virus is divided into 3 main genera: influenza virus A, B, and C.

• Influenzavirus A is the most common genera and can be found infecting humans, animals, and birds. The following serotypes within this genera have been confirmed in humans: H1N1, H2N2, H3N2, H5N1, H7N7, H1N2, H9N2, H7N2, H7N3, H10N7, and H7N9. There are potentially 144 different subtypes of influenza A virus possibilities.
• Influenzavirus B almost exclusively infects humans and does not branch into multiple subtypes. It mutates much slower than subtype A, and consequently is less genetically diverse. The low genetic diversity plus the small host range makes it so that pandemics of subtype B do not happen. However, it still mutates enough that life-long immunity is not possible.
• Influenzavirus C is less common and tends to only cause mild disease, and is therefore understudied.

The flu virus is constantly changing and mutating.

There are two ways in which the flu virus mutates from year to year. The first is called antigenic drift. When you think of the word “drift” you may imagine a slow flow from on thing to another. These slow changes in the influenza virus occur over the year(s) and create just enough change in the virus so that your immune system can no longer recognize it from one year(s) to the next. When one flu virus drifts and mutates in to an unrecognizable form, these are considered two different “strains” of the virus.  This is the reason why we have a new flu vaccine every year.

The second way the virus mutates occurs more suddenly, and we call these changes antigenic shifts. This happens when two different strains of the virus infect the same cell and then combine to form a new version of the virus. The mutation may create a new flu subtype and because people have no immunity to the new subtype, it can lead to a severe flu epidemic or pandemic. This is also the type of mutation that allows the virus to jump between species–animal to human–such as in the case of the 2009 Swine flu pandemic.

It’s because of these genetic changes that the influenza virus is able to display different versions of HA and NA on its surface and is able to keep eluding the immune system year after year.

As always, thanks for reading!

Resources:

1. Hillcrest Medical Center. www.hillcrest.com

2. www.Flu.gov

3. Sino Biological Inc.  www.sinobiological.com