September 25, 2023

What You Need To Know About Flu Strains

There are three antigenic types of influenza, but A and B are responsible for our flu season

FLu vaccine vials with needle filled with liquid on a blue background.

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:


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  • Proper handwashing.
  • Avoiding touching your eyes, nose and mouth.
  • Avoiding people who are sick.
  • Wearing masks in situations when you can’t avoid people who are sick, such as caring for a sick child.

We talked with infectious disease doctor Sherif Mossad, MD, about the 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.”

What are the strains of flu?

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

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

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.

Flu naming conventions

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:

  • The antigenic type (A, B, C or D).
  • The host of origin (like swine, equine, chicken, etc.). For human-origin viruses, no host of origin designation is given.
  • Geographical origin.
  • Strain number.
  • Year of collection.
  • For influenza A viruses, the name includes a number value for two proteins found on the virus’ surface. They’re hemagglutinin (HA) and neuraminidase (NA). For example, influenza A(H1N1) virus and influenza A(H5N1) virus.
  • The 2009 pandemic virus was assigned a distinct name: A(H1N1)pdm09 to distinguish it from the seasonal influenza A(H1N1) viruses that circulated prior to the pandemic.
  • When humans are infected with influenza viruses that normally circulate in pigs, these viruses are called variant viruses and are designated with the letter “v” (for example, an A(H3N2)v virus).

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.

How does the flu mutate every year?

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

Antigenic driftrefers 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 shiftis 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.”

How does the flu vaccine work?

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.

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.

Protect yourself from the flu

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

  • Practice good hand-washing hygiene. Wash your hands thoroughly with soap and water. If you aren’t able to use soap and water, use an alcohol-based hand sanitizer.
  • Avoid being around other people when you don’t feel well, especially when you have a fever.
  • Avoid being around people who are sick whenever possible.
  • Avoid touching your eyes, nose and mouth.
  • Eat a healthy diet, exercise regularly and get enough sleep.

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