Understanding the Different Types of Flu

What we all know as "the" flu is actually a collection of influenza virus strains that mutate (change) over time. Protecting yourself from these potentially dangerous viruses is best done by getting the flu vaccine and practicing everyday prevention techniques, like proper hand washing and avoiding sick people.

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We talked with infectious disease doctor Sherif Mossad, MD, about the many strains of flu and how you can keep from getting sick this flu season.

"What all flu strains have in common are the symptoms they cause," Dr. Mossad says. "Fever. Headache. Cough. Those are the three cardinal manifestations of all flu viruses. Flu can cause stomach issues for some people, too, but ‘stomach flu’ is really a misnomer."

There are three types of flu viruses that affect people. They’re known as influenza viruses A, B and C. Influenza A and influenza B viruses are the heavy hitters. They’re the ones that cause the most severe illness and lead to widespread outbreaks. Influenza C viruses cause mild illnesses in people. They’re more similar to a common cold, Dr. Mossad says, and they aren't detected by flu tests.

(Fun fact: There is an influenza D virus, but it's mostly found in cattle and hasn't been known to affect people.)

When people talk about the flu, it's really influenza A and B viruses. They’re the viruses that are responsible for our annual flu season.

Influenza A viruses are the most common flu viruses. They’re the cause of regular seasonal flu outbreaks, as well as global flu pandemics. Influenza A viruses can affect both people and animals.

The 1918 pandemic and the 2009 H1N1 pandemic (what some people call the "swine flu"), for example, were Influenza A virus outbreaks.

Dr. Mossad says influenza A virus outbreaks usually occur earlier in the flu season. In the Northern Hemisphere, influenza A is most likely to make its rounds from October to March.

Common strains of influenza A include the H1N1 and H3N2 varieties.

Influenza B viruses affect only people, not animals, and they don't spread to the point of pandemics. Influenza B viruses are more likely to make you sick later in the flu season, from about January to May in the Northern Hemisphere.

There are so many different strains of flu viruses that the World Health Organization (WHO) created an internationally accepted naming convention to help track them.

The name of each virus is made up of:

So, for example, the A/Sydney/05/97(H3N2) virus is an influenza A virus that originated in Sydney, is strain number five, was collected in 1997 and is an H3N2 subtype.

Long story short: There are boatloads of flu strains out there. And they’re constantly changing and creating new viruses.

Dr. Mossad explains there are two ways the flu virus mutates.

Antigenic drift refers to the small ways in which the flu virus mutates each year. Antigenic drift is the reason for annual flu epidemics. So, even if you had one strain of H3N2 last year, it's now a little different, and you can get it again this year.

Antigenic shift is a tidal wave of flu virus mutation.

"Antigenic shift is when viruses change so much that you wind up with a virus that humans have never been exposed to at all," Dr. Mossad says. "That's what happened in 2009 with the H1N1 swine flu virus. It was an entirely new virus to humans, so no one had built up any immunity to it at all."

Each year, the WHO and the U.S. Centers for Disease Control and Prevention (CDC) monitor trends to determine the strains of flu that’ll be most common. And each year, the flu vaccine is created to protect you from what these experts expect will be the two most prevalent influenza A viruses and two most prevalent influenza B viruses for that flu season.

For the 2022 flu season, the U.S. Food and Drug Administration (FDA) determined that egg-based influenza vaccines (the most commonly used vaccine method) would be formulated for these viruses:

Remember, the flu virus mutates. Fast. So getting a flu shot every year is the absolute best method to protect yourself from the strains that are likely to make their rounds this season.

In addition to getting your flu shot every fall, Dr. Mossad suggests these best practices to stay healthy this flu season: