The fall and winter months bring crisp air, cozy sweaters, and holiday festivities. They also bring the flu—a contagious respiratory illness caused by influenza viruses that infect the nose and throat and in some cases, the lungs.

For many people, the flu is a mild illness with a fairly swift recovery. But for others, particularly older adults and those with chronic health conditions, it can become severe and even life-threatening. According to the Centers for Disease Control and Prevention (CDC), each year during the period 2010-2023, flu has caused:

• Up to 41 million illnesses
• Up to 710,000 hospitalizations
• Up to 51,000 deaths¹

You can protect yourself and those you care about by learning more about the flu, its symptoms, and how to limit your risk of becoming infected.

What are the different types of flu?

Flu viruses are divided into four types: A, B, C, and D. During flu season (October through May), you’ll mostly hear about flu A and flu B. Flu C viruses only cause mild illness in humans and are not linked to large outbreaks. Flu D viruses do not appear to infect humans at all and are mainly found in cattle.

Now that we know there are two main types of flu viruses that affect humans, you may be wondering: Which flu is worse, A or B? Answering that question starts with learning more about their differences and how they affect us.

What is the difference between influenza A and B?

While flu A and flu B both cause seasonal flu, there are several differences in terms of their structure, behavior, and who they impact.

Flu A is the more common of the two main flu types, making up about 75% of all cases.² This type can infect not only humans, but also animals such as birds and pigs.

The Flu A virus is divided into subtypes based on two proteins found on its surface. These subtypes can combine in different ways to form various strains of flu A—like H1N1 and H3N2, which are the two main strains that currently affect humans. This versatility allows flu A to mutate and change more frequently, making it harder for our immune system to recognize and fight off.

Since flu A can infect multiple species and mutate more easily, it’s often the culprit behind widespread flu outbreaks and pandemics. This includes the 2009 H1N1 pandemic (also known as “swine flu”), which affected millions of people around the globe. Every year leading up to flu season, health experts keep a close eye on flu A strains. This scrutiny enables scientists to develop a vaccine that effectively targets the strains most likely to be circulating.

Flu B only affects humans. It’s less common, making up roughly 25% of all flu cases, and less likely to cause pandemics (although it can still cause outbreaks).

Flu B viruses are broken down into two main lineages: the B/Yamagata and B/Victoria lineages. These two strains can still cause seasonal flu epidemics, but they’re less likely to mutate. Also, they don’t change as much from year to year like flu A strains do.

Another difference between flu A and flu B timing. Flu B tends to circulate later in the flu season compared to A, often peaking in the spring.

Are the symptoms of flu A and B different?

Flu A and B share similar symptoms. The most common symptoms of flu include:

• Fever (100-102 °) and/or chills
• Headache
• Muscle or body aches
• Weakness/fatigue
• Extreme exhaustion

Other flu symptoms may include stuffy/runny nose, sneezing, and sore throat.

Which flu is worse, A or B?

Flu A tends to be more aggressive and cause more severe symptoms compared to flu B. It’s more common in adults. In contrast, flu B is more common in children. While it typically causes mild illness, type B can be more severe in children under age 5 and older adults.

Generally speaking, older adults are more likely to experience complications from both types of flu viruses. This is because our immune system is not as efficient at fighting off viruses as we age.

Can you have flu A and B at the same time?

Yes, it's possible to have flu A and flu B and even other respiratory illnesses (like COVID) at the same time. However, it’s not yet understood how common this is. You can also contract flu B after getting flu A (or vice versa), since having the antibodies from one type does not protect you from getting infected with the other.

How do I test for influenza A or B?

Most flu tests, including rapid influenza diagnostic tests (RIDTs) and more advanced lab tests, can detect both flu A and flu B. Whether you take the test at home or at your doctor’s office, the results will help your provider determine which type of flu you have and what treatment you need, if any.

What medicine is given for flu A or B?

Treatment is the same no matter which type of flu you have. Your doctor may prescribe antiviral drugs, which can help reduce the severity and duration of symptoms when taken early enough. Antiviral treatments are especially beneficial for older adults and people with underlying health conditions, since they can help prevent serious complications from the flu.

Supportive care can help your recovery as well—such as rest, drinking plenty of fluids, and taking over-the-counter medications to relieve fever and aches.

How do I prevent flu A and flu B?

No matter if it’s flu A or flu B, the best way to protect yourself is by getting a flu vaccine each year. The flu vaccine is designed to guard against the most common strains of both types expected to circulate that season. For older adults, the CDC recommends higher-dose flu vaccines (including high-dose inactivated and recombinant) or the adjuvanted inactivated flu vaccine over standard-dose unadjuvanted flu vaccines—since these can provide better protection and a stronger immune response.

Timing is important when it comes to getting your flu shot, according to Dr. Robert Hopkins, Jr., medical director for the National Foundation for Infectious Diseases. “For older adults and for folks who’ve got chronic medical problems, I usually recommend getting the flu vaccine around anytime from the middle of September through early October,” he explained to Fortune Well. “Because in those of us with immune systems that aren’t quite as robust, we want to make sure that we vaccinate at a time that’s likely to give us immune protection throughout the flu season.”

In addition to getting vaccinated, you can protect yourself from both flu A and flu B by washing your hands frequently, avoiding close contact with people who are sick, and wearing a mask in crowded or indoor settings during flu season. It's also a smart idea to clean and disinfect frequently touched surfaces at home and at work.

Find nearby flu vaccination sites by entering your ZIP code at Vaccines.gov. Good news: All recommended vaccines are now covered by Medicare and Medicaid—so you’ll pay nothing out of pocket at your visit.

Sources

1. Centers for Disease Control and Prevention (CDC). About Estimated Flu Burden. Found on the internet at https://www.cdc.gov/flu-burden/php/about/index.html

2. Yellanthoor Ramesh Bhat. Influenza B infections in children: A review. World Journal of Clinical Pediatrics. November 19, 2020. Found on the internet at https://www.wjgnet.com/2219-2808/full/v9/i3/44.htm