There are several types of influenza viruses, and just like we have seen with COVID-19, they can change and mutate over time. If you catch the flu one year, you can catch it again the next year – because it may be a different virus that is circulating, or the virus has changed enough that your body doesn’t recognize it and your antibodies don’t work against this new or mutated version.
There are three types of influenza viruses that affect humans:
- Type A: Type A influenzas affect both people and animals, such as birds. The animals help spread the virus. Type A cases of influenza cause most flu pandemics or epidemics. In 1918, the world was hit by a flu outbreak that killed millions of people. Experts were worried that the H1N1 virus in 2009, what was called “the swine flu” by many, would have similar outcomes. Luckily, it did not. The virus did spread rapidly, but it was not as deadly as feared.
- Type B: Type B influenzas do not infect animals and do not cause epidemics or pandemics, but they can still cause serious harm to humans.
- Type C: Type C influenzas are milder than either types A or B. They do not cause epidemics or pandemics, and they only affect humans.
Influenza A types, such as H1N1, are named by their proteins, hemagglutinin (“H”) and neuraminidase (“N”). There are 16 H proteins and 11 N proteins in all. The virus name depends on which hemagglutinin and neuraminidase are present in the virus. You can have any combination, such as H6N2 or H15N9.
Seasonal influenza, caused by both type A and B, is the most commonly known flu type. The severity of illness and the number of deaths caused by the flu varies quite a bit from year to year, depending on how strong the virus is. For example, an estimated 34,200 people in the U.S. died during the 2018/19 flu season, according to the Centers for Disease Control and Prevention (CDC). The year before, there were 61,000 flu-related deaths. A combination of factors lead to the number of deaths, including how well the virus is transmitted, how many people are vaccinated, and how effective the vaccine is for that year’s main flu strain.