As you probably know, the influenza virus is constantly changing. This is why there are new vaccines created each year. The flu virus can change in two separate ways.
One way the flu virus can change is called the “antigenic drift.” This drift refers to very small changes in the genes of the influenza virus. These small changes happen constantly over time, when the virus replicates.
Generally, the small changes will produce viruses that are very similar to their predecessors, as they are all closely related. Because these viruses are related, they share the same “antigenic” properties. If your immune system is exposed to a virus similar to one it has already been exposed to, or vaccinated for, it will respond in kind. In other words, you won’t be susceptible to it.
However, because these small changes build up over time, they become more and more different as time passes. As the viruses become dissimilar, the body won’t recognize them, so you’ll need a new vaccination.
How it Works
When someone is infected with one strain of the flu virus, they will then develop an antibody against that strain. Then, as those small antigenic changes accumulate, the antibodies which were created against the older strains will no longer recognize the new strains of the virus. This means they will “catch” the flu again and get sick.
This is typically why we catch the flu over and over again, and this is why new vaccines must be created and reviewed each year.
The other way the flu virus changes is through an “antigenic shift.” The shift is a more abrupt, major change in the flu virus, rather than the small “drift” mentioned above. This major change results in new proteins in the virus which will infect us.
This shift creates completely new sub types of the virus, like those found in animals. These flu virus strains are so different from the ones we’ve been infected by in the past that we will not have immunity to them.
There was a shift like this in 2009 when the H1N1 virus was found and spread so quickly that it caused a pandemic. Vaccines had not yet been prepared for the outbreak, because it was not yet recognized.
Generally when a shift happens, there is little or no protection against the new virus simply because it is a new strain.
Even though antigenic drifts are happening all the time in the flu virus, an antigenic shift happens only once in a while. It is important to get a new vaccine each year, though, to protect yourself from these changes.