Key takeaways:
There are four main strains or types of influenza virus: A, B, C, and D.
Influenza A and B viruses cause the flu season each year.
Flu virus strains are always changing. That’s why the flu vaccine changes every year too.
To protect yourself from getting sick with the flu, get the yearly flu vaccine and take other preventive measures, like frequently washing your hands.
The influenza (flu) virus is a type of virus that mainly spreads in air droplets and on surfaces. Once the virus enters your mouth, nose, and lungs, it can lead to flu symptoms. Flu symptoms include fever, chills, and body aches. They also include cough, runny nose, and sore throat. In some cases, the flu can even be deadly.
There are several different types of flu, but only certain ones are likely to cause an infection. Read more about the different types and why getting the flu shot is the best way to protect yourself from getting sick with the flu this year.
What are the different types of flu?
There are four main types of flu. Influenza A and B are the ones that cause seasonal flu. Here’s a closer look at each type.
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Influenza A
Influenza A is active during the flu season every year. In the past, certain strains of influenza A have caused flu pandemics, including the 1918 "Spanish flu" and the 2009 H1N1 "swine flu" pandemics. The yearly flu shot has protection against the two most common strains of influenza A.
There are different subtypes of influenza A. The name of each subtype is based on the types of hemagglutinin (H) and neuraminidase (N) that strain of virus carries. Hemagglutinin and neuraminidase are proteins on the outside of the virus. Researchers use these proteins like name tags to tell the subtypes apart. For example, the H1N1 subtype of influenza A carries hemagglutinin type 1 (H1) and neuraminidase type 1 (N1) proteins.
Influenza B
Influenza B is also part of the flu season every year, but it doesn’t tend to cause pandemics. The yearly flu shot also has protection against influenza B.
There are also different varieties of influenza B. These are called “lineages” and are named after the geographical region where they were first discovered. The B/Victoria and B/Yamagata lineages of influenza B have been active in recent years.
Influenza C
Influenza C causes mild symptoms and doesn’t usually spread as part of flu season. So the yearly flu shot doesn’t protect against influenza C.
Influenza D
Influenza D mostly causes disease in other animals, especially cows. It doesn’t cause symptoms in humans, so protection against it isn’t included in the flu shot.
Could you have the flu? From nasal congestion to muscle aches, read about the most common flu symptoms to watch out for.
Getting the flu shot: Learn how the flu shot works, what it’s made of, and why you should get one every year.
Over-the-counter (OTC) flu treatments: Learn how to choose the best OTC flu medications to help relieve symptoms fast.
Symptoms of the flu
Both influenza A and B cause seasonal flu. The symptoms of both types are similar, but influenza A can sometimes be more severe. Common symptoms include:
Fever
Cough
Runny nose or nasal congestion
Muscle aches
Feeling tired
What types of flu are expected to show up this season?
The strains of the flu virus in circulation vary at any given time. The World Health Organization’s Global Influenza Surveillance and Response System (GISRS) has been monitoring flu activity around the world since 1952. The CDC FluView Interactive surveillance platform reports the activity of different influenza viruses in the U.S. each week.
- TamifluOseltamivir
- AmantadineGeneric Symadine and Symmetrel
- XofluzaBaloxavir Marboxil
Based on all the available data, the FDA Vaccines and Related Biological Products Advisory Committee chooses which influenza strains to include in the flu vaccine for the upcoming flu season. The strains are chosen based on which flu viruses are currently circulating and which ones are most likely to cause problems that winter.
The 2025-26 flu vaccine will contain protection against two strains of influenza A (H1N1 and H3N2) and one strain of influenza B (from the B/Victoria lineage).
How effective is the flu shot?
The effectiveness of the flu shot is different each year. Between 2009 and 2020, the flu shot was, on average, around 40% effective in preventing the flu.
In 2024-25, it was about 35% to 55% effective in adults. The flu shot doesn’t have a higher prevention rate because the influenza virus is always changing. This is why people don’t have lifelong immunity to the flu after getting sick with it. The body’s immune system may not be able to use antibodies made for a previous flu infection for a different flu virus strain.
Since the flu virus changes each year, the vaccine does as well. Sometimes, the flu virus strains that are chosen for the vaccine don’t completely match the virus strains that are most active that year. But even when the vaccine isn’t a perfect match, it still lowers your chances of getting sick. And it makes your symptoms less severe if you do catch the flu.
How to best protect yourself from the flu
Everyone should take steps to prevent getting and spreading the flu. This keeps you healthy as well as the people in your community. Some people have a higher risk of flu complications, including young children, older adults, and people with certain medical conditions such as asthma and HIV.
Here are some ways to prevent the flu:
Get your yearly flu vaccine.
Wash your hands frequently with soap and water. You can also use alcohol-based hand sanitizer if you can’t wash your hands right away.
Cover your mouth when coughing and sneezing, preferably in your elbow. Throw away any tissues immediately.
Avoid touching your eyes, nose, and mouth.
Disinfect surfaces.
Avoid contact with people who are sick.
Limit contact with others if you’re sick.
Frequently asked questions
Bird flu (avian influenza) describes a group of influenza A viruses that spread among wild birds. Poultry that get infected with bird flu usually get very sick and die from the infection.
People rarely get infected with bird flu, but it can happen. It’s more likely when someone is exposed to an infected bird or animal. Symptoms of bird flu can range from mild ones (like cough and sore throat) to more severe ones (like trouble breathing and high fever).
Influenza A is most contagious during the first 3 days of the illness, but you can still spread it even as your symptoms are getting better. You’re no longer contagious once the symptoms are gone (usually 5 to 7 days). Other types of flu are contagious for about the same amount of time.
In general, influenza A can feel worse because it can cause worse symptoms (like a higher fever or a more intense cough). But any flu infection can be more severe in people with certain risk factors, like those older than age 65 and those who have a weakened immune system.
Bird flu (avian influenza) describes a group of influenza A viruses that spread among wild birds. Poultry that get infected with bird flu usually get very sick and die from the infection.
People rarely get infected with bird flu, but it can happen. It’s more likely when someone is exposed to an infected bird or animal. Symptoms of bird flu can range from mild ones (like cough and sore throat) to more severe ones (like trouble breathing and high fever).
Influenza A is most contagious during the first 3 days of the illness, but you can still spread it even as your symptoms are getting better. You’re no longer contagious once the symptoms are gone (usually 5 to 7 days). Other types of flu are contagious for about the same amount of time.
In general, influenza A can feel worse because it can cause worse symptoms (like a higher fever or a more intense cough). But any flu infection can be more severe in people with certain risk factors, like those older than age 65 and those who have a weakened immune system.
The bottom line
Of the four different types of flu virus, influenza A and B are most likely to cause problems during flu season. The influenza virus changes frequently, so the flu shot is redesigned each year to target the strains most likely to make you sick.
Getting a yearly flu shot is the best way to keep yourself safe from the flu. Preventive measures, such as washing your hands and avoiding people who are sick, can also help.
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References
Bhat, Y. R. (2020). Influenza B infections in children: A review. World Journal of Clinical Pediatrics.
Centers for Disease Control and Prevention. (n.d.). FluView.
Centers for Disease Control and Prevention. (n.d.). National, regional, and state level outpatient illness and viral surveillance.
Centers for Disease Control and Prevention. (n.d.). Pandemic flu.
Centers for Disease Control and Prevention. (2024). About bird flu.
Centers for Disease Control and Prevention. (2024). Effectiveness against different flu viruses.
Centers for Disease Control and Prevention. (2024). How flu viruses can change: "Drift" and "shift".
Centers for Disease Control and Prevention. (2024). Key facts about seasonal flu vaccine.
Centers for Disease Control and Prevention. (2024). People at increased risk for flu complications.
Centers for Disease Control and Prevention. (2025). About influenza A in animals.
Centers for Disease Control and Prevention. (2025). Backyard flock owners and bird flu.
Centers for Disease Control and Prevention. (2025). CDC seasonal flu vaccine effectiveness studies.
Centers for Disease Control and Prevention. (2025). Types of influenza viruses.
Frutos, A. M., et al. (2025). Interim estimates of 2024–2025 seasonal influenza vaccine effectiveness — Four vaccine effectiveness networks, United States, October 2024–February 2025. Morbidity and Mortality Weekly Report.
Taubenberger, J. K., et al. (2020). The 1918 influenza pandemic and its legacy. Cold Spring Harbor Perspective in Medicine.
U.S. Food and Drug Administration. (2019). Vaccines and related biological products advisory committee.
U.S. Food and Drug Administration. (2025). Influenza vaccine composition for the 2025-2026 U.S. influenza season.
World Health Organization. (n.d.). Global influenza surveillance and response system (GISRS).
