Influenza A and B are both highly contagious. Symptoms are similar: fever, headache, cough, sore throat, muscle ache, shortness of breath, vomiting and more.

Published: November 20, 2023 | Updated: September 19, 2024

There are two types of flu viruses that make people sick each year, influenza A and influenza B. Influenza A viruses are also present in animals. Influenza B viruses are only present in humans. 

Influenza A viruses are more common than influenza B among adults, and it is influenza A that causes seasonal flu epidemics most years in the United States.

Influenza B viruses also can cause seasonal epidemics, but influenza A viruses are the only ones that can cause a pandemic, which is a global spread of disease. That’s because Influenza A viruses can mutate — or change — more rapidly than influenza B viruses.

Both influenza A and influenza B are highly contagious, and their symptoms are similar: fever, headache, cough, sore throat, muscle ache, shortness of breath, vomiting and diarrhea. Untreated, flu symptoms can last for weeks. 

The flu also can cause severe illness and make some chronic medical conditions, such as asthma, heart disease and diabetes, worse. In some cases, the flu can lead to death.

Influenza A is generally considered worse than type B influenza among adults, though symptoms vary from person to person. Most adults have built up immunity against type B influenza. Because they are different, it’s possible to be infected with both flu A and flu B at the same time. 

Influenza B symptoms can be severe in children. Children under 5 are at higher risk of serious flu complications. Children under 2 are at the highest risk for serious complications. 

Flu viruses spread when a person who is infected sneezes or coughs and droplets travel to another person’s nose, mouth, or eyes.

Thorough and frequent hand-washing is one important way to protect yourself against infection. 

The best protection is a flu shot. The Centers for Disease Control and Prevention (CDC) recommends everyone 6 months and older receive a flu vaccine. All 2023-24 flu vaccines are quadrivalent, meaning they provide protection against four types of flu, two types of influenza A and two types of influenza B.

“The seasonal flu vaccine can help prevent you from getting sick and also can be effective at keeping you from developing severe symptoms,” said Mary Rademaker, M.D., medical director for Norton Immediate Care Centers. “This is especially important for those who have cancer, survived cancer or have another condition that has weakened your immune system.”

The seasonal flu vaccine allows your body to build up immunity against the flu without getting sick. The flu shot contains dead influenza virus, which activates your immune system to gear up against the real thing if you get an influenza virus infection. Dead flu virus won’t give you the flu, however some people feel sluggishness and other side effects as their immune system activates after the vaccine.

It’s important to get a flu shot every year because strains of the flu mutate over time. Getting a shot each year gives your body immunity to the latest strains of the influenza virus. 

The CDC picks which strains of type A flu and type B flu each year depending on which flu viruses are making people sick prior to the upcoming flu season, how quickly the virus is spreading, whether the previous year’s vaccine will protect against those flu viruses and whether the vaccine could protect against multiple strains of the virus.

If you get the flu, your health care provider will help you decide how best to care for you based on test results determining which type of flu you have, your medical history and your symptoms. Some antiviral medications work better against type A flus, while others work better against type B. 

Antiviral treatment is recommended as soon as possible — ideally within 48 hours — for anyone hospitalized with suspected or confirmed influenza; anyone who has severe, complicated or progressive flu; or anyone at higher risk for complications from the flu.