VACCINE RESOURCE COMMITTEE

of the Edmonton Zone Medical Staff Association

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Influenza Vaccine Question #4:

Why is the influenza vaccine needed every year?

Influenza viruses are always changing. They change through two processes: “antigenic drift” and “antigenic shift”.

The virus is always changing, resulting in a new vaccine needed for the new strains.

Antigenic Drift

This is similar to the game “Broken Telephone”. A person will start with one sentence, but as the sentence is whispered around the circle of people, small changes and errors creep in. By the time the sentence gets back to the person who started, it may be similar to the sentence at the beginning, but not entirely the same. This process is similar to how antigenic drift works for influenza viruses.

As the influenza virus spreads from one person to another, it is always making copies of itself. As it continually replicates, small changes to its genes can happen over time. As more changes accumulate, the harder it becomes for the immune system to recognize, and protect against, the newer versions of the virus. This process is called “antigenic drift”. This is the main way influenza viruses change over time. It is also one of the main reasons for annual influenza vaccines.

Antigenic Shift

This time, in the Broken Telephone game, someone decides to completely change the language the game is being played in - midway through the circle. Because the person who made the change is the only speaker of that language in the circle, the sentence then becomes unrecognizable to everyone else playing. This version of the game is similar to how antigenic shift works for influenza viruses.

Antigenic shift occurs when two different influenza strains, one native to humans and one native to animals (commonly birds), swap genetic material. This swap creates an entirely new influenza strain that has the potential to infect humans. Since this influenza strain would be completely novel to our bodies, our immune systems would not have any memory of how to fight it. This is why antigenic shift can be so dangerous, and why it has the potential to cause pandemics. Luckily, antigenic shift does not happen often, and influenza pandemics are rare.

Sources:

Immunize Canada; Why do we need to get a fl u vaccine every year? Answer: drift and shift.