Every year, we talk about the flu as if it were a single, solitary thing. However, the flu is not something that ever stays the same: flu outbreaks are never consistent because influenza viruses are always evolving. Epidemiologists expect slight changes with each outbreak, but sometimes they are caught off guard by such a radical change that we are left unprepared and vulnerable to pandemic. This is when the flu becomes more dangerous. The reasoning is simple; from year to year, the viruses change in two different ways; antigenic drift and antigenic shift.
Antigenic drifts are small changes in the genes of the flu viruses that happen continually and gradually over time. Each time the virus replicates, it changes. The small genetic changes produce viruses that are similarly related to one another but not exactly the same. These viruses share the same antigenic properties, and an immune system exposed to a virus that is similar can usually recognize it and respond accordingly. Antigenic drift changes are like the flu kicked up a notch, but they are not anything that will knock your immune system over the head with a club.
These minute genetic changes can build up over time and eventually result in viruses that are very different from the original. When that happens, the body’s immune system does not recognize those viruses and is vulnerable.
For example, a person infected with a particular strain of flu virus develops antibodies against that specific virus. As antigenic changes accumulate, resulting in a virus with very different antigenic properties, the same person is just as susceptible to infection as anyone else is as the antibodies created against the old virus no longer recognize the newer one. This is why the flu vaccine gets modified each year. The good thing about antigenic drift is that it is slow and gradual.
The second type of change in the flu virus is called antigenic shift. Antigenic shift is an abrupt, major change in the influenza A virus, resulting in new proteins present in influenza viruses that infect humans.
Antigenic shift results in a new influenza A subtype or a virus that has emerged from an animal population that is so different from the same subtype in humans that most people do not have immunity to the new virus. The H1N1 virus outbreak of 2009 was the result of an antigenic shift. A new combination of genes emerged to infect people and quickly spread, causing a pandemic. When an antigenic shift happens, most people have little or no protection against the new virus. It’s fast, unexpected and very dangerous. Antigenic shifts make it harder to protect ourselves against the flu virus.
While influenza viruses change almost constantly by antigenic drift, antigenic shift happens infrequently and without warning. Type A viruses undergo both kinds of changes; influenza type B viruses change only by the more gradual process of antigenic drift. Either way, the CDC recommends getting the flu shot every year. While it might not be able to protect us from the rare radical antigenic shift, it can help protect us from viruses that undergo the more common antigenic drift, which will leave our immune system less vulnerable to one that has undergone an antigenic shift.