Mission: To save lives and reduce hospitalizations by protecting children, families, and communities against influenza.
Insights on Influenza
  • Flu Facts

What Are the Different Types of Influenza Viruses?

flu virus anatomy
Posted on January 6, 2020
Share this article:

Some people may think influenza (flu) is just a standard virus that comes around every year. But there are actually four different types of flu viruses, including A, B, C, and D. 

Influenza A: Influenza A viruses can be found in many different animals, including birds and mammals, as well as humans. These viruses are characterized by surface proteins, including hemagglutinin (“H”) and neuraminidase (“N”). These viruses are further separated into subtypes by number. There are 18 different H subtypes and 11 different N subtypes, and 131 subtype combinations have been detected in nature. Examples of flu viruses in this category include H1N1 and H3N2.

Influenza B: These viruses only affect people and are characterized according to where the virus was originally isolated. The two main categories (lineages) of influenza B include B/Yamagata and B/Victoria. 

Influenza C: This type of flu can affect people, but most often only causes mild illness. Influenza C infections are not thought to cause human flu epidemics, which are defined as widespread flu illness in a localized area. 

Influenza D: This type of flu primarily affects cattle and is not known to affect people. 

How can flu viruses change?

Flu viruses are constantly changing, which is why flu vaccine composition changes every year to match the circulating strains. Flu viruses can change slightly (know as “antigenic drift”) or rapidly (known as “antigenic shift”). Antigenic drift refers to small changes in the genetic make-up of flu viruses that can lead to changes in the surface proteins (hemagglutinin and neuraminidase). Antigenic shift refers to an abrupt, major change in an influenza A virus, resulting in new surface proteins that can infect humans.    

Flu viruses are constantly changing, which is why flu vaccine composition changes every year to match the circulating strains.

What’s the difference between “seasonal flu” and “pandemic flu”?

Influenza A and B viruses affect people and cause what is commonly referred to as “seasonal flu”, which occurs every year. Influenza A also has pandemic potential, meaning that new (novel) influenza A viruses can emerge and cause a pandemic, defined as a global outbreak. For a pandemic to occur, the new virus has to be able to infect people easily and spread efficiently from person to person. There have been four flu pandemics in the past 100 years. 

How are flu strains selected for annual flu vaccines?

There are over 100 centers in the world that conduct year-round, global surveillance on circulating flu strains. These laboratories send representative flu virus samples to five collaborating World Health Organization (WHO) centers located in the United States, United Kingdom, Australia, Japan, and China. Using this information, the WHO then recommends flu vaccine composition twice a year: once in February for the upcoming season in the Northern Hemisphere and a second time in September for the upcoming season in the Southern Hemisphere. Following WHO’s recommendations, each country then makes their own decision about which flu strains to include in the annual vaccine. In the U.S., this decision is made by the Food and Drug Administration (FDA). Annual flu vaccines are designed to protect against three or four different flu strains, including influenza A and B types. 




You might also be interested in...

Prevent winter flu with these tips.
7 Ways to Keep Your Family Healthy this Winter
Flu Essentials: Navigating the Influenza Season with Confidence Part I
Going Home for the Holidays? Flu Prevention is Key to Ensure Your Family is Protected
Stay Informed
Sign up for our Weekly Flu News and quarterly newsletter, Fighting Influenza, to stay up-to-date on the latest flu news.


You are required to accept all of the terms and conditions in this License Agreement in order to receive a limited, non-exclusive, non-transferable, non-assignable, royalty-free license to use the FAMILIES FIGHTING FLU, INC. logo, displayed below (the “FFF Logo”), from Families Fighting Flu, Inc. (“FFF”). In order to accept these terms and conditions, you must click the “I Accept” button below.

  1. FFF hereby grants to you, and you accept, a limited, non-exclusive, non-transferable, non-assignable, royalty-free license to use the FFF Logo, which you may download and use solely in online and print earned media publications for a period of no more than six (6) months in the United States.
  2. You acknowledge that the FFF Logo is a valid trademark; that FFF is the sole owner of the FFF Logo; and that FFF retains all right, title, and interest in and to the FFF Logo. You agree that any and all use that you make of the FFF Logo, and any and all goodwill arising from your use of the FFF Logo, shall inure exclusively to the benefit of FFF.  You also agree that you shall not challenge the validity of the FFF Logo or FFF’s ownership or title thereto during the term of the License Agreement or thereafter.
  3. You agree that you shall use the FFF Logo only in the form shown above, and that you shall not alter, modify, change, or edit the FFF Logo in any manner.
  4. You acknowledge the valuable goodwill and reputation in the FFF Logo, and that it is essential that the high standards and reputation associated with the FFF Logo be maintained. You therefore agree to use the FFF Logo consistent with the highest standards of quality so as to protect and maintain the FFF Logo and FFF’s rights therein.
  5. FFF shall have the right to review and inspect your use of the FFF Logo at any time. FFF, in its sole discretion, may determine that you are not using the FFF Logo in an acceptable manner and require that you discontinue your use of the FFF Logo.  To the extent that FFF requires that you discontinue your use of the FFF Logo, you agree that you shall remove the FFF Logo from all online and print earned media publications as soon as practicable under the circumstances, and that you shall not use the FFF at any time in the future.
  6. You must include the following notice in connection with any use of the FFF Logo: “The Families Fighting Flu, Inc. logo is the registered trademark of Families Fighting Flu, Inc.”
I Accept