What’s the Difference Between Influenza A and Influenza B?

There are some things you can count on as fall turns to winter. The days get shorter. There’s a chill in the air. And chances are good that a respiratory flu virus is beginning to spread.

Flu season in the United States typically starts in October and tapers off in late April. And there are two main strains of the influenza virus that cause our annual flu season: influenza A and influenza B.

If flu acted like a two-person relay race, you’d typically see influenza A at the starting block in autumn, and hitting its full stride by February. Come March, influenza B picks up the viral baton.

That said, the flu season can vary from year to year. For example, data from the U.S. Centers for Disease Control and Prevention (CDC) show the 2024-2025 flu season got a late start. The number of reported cases didn’t start climbing until late November.

Both virus strains have some characteristics in common, but there are also some key differences. Infectious disease doctor Sherif Mossad, MD, explains what sets them apart and why those differences matter.

What are some differences between type A and type B flu?

Influenza A and influenza B have a lot in common. But there are some overarching differences between the two flu types. For example, influenza A usually accounts for more flu cases and causes more severe illness than type B flu. For example, flu A accounted for 96% of flu cases reported in late 2024.

Another difference is how the flu types affect kids. “In children, influenza A is associated with more ear infections than influenza B, while influenza B is associated with more seizures, vomiting and diarrhea than influenza type A,” says. Dr. Mossad.

But the single most important difference is this: Influenza B flu only infects humans. Influenza A, on the other hand, can infect birds and mammals. This explains why antigenic shift can occur with influenza A, but not influenza B.

Antigenic shift and antigenic drift

“Antigenic shift occurs when bird or animal influenza A is transmitted to humans. This results in pandemics, since humans don’t have specific immunity to these viral serotypes (groups of viruses),” shares Dr. Mossad.

Antigenic shift is the reason why an influenza A can cause a pandemic. Both influenza A and influenza B can cause annual epidemics. Epidemics are declared when there are more cases of a specific disease than usual in a specific group of people. A pandemic happens when an epidemic affects thousands or even millions of people in several different countries or continents.

The 2009 A/H1N1 pandemic is an example of antigenic shift in action. In that case, an influenza A virus in pigs jumped to humans.

That leap launched the 2009 H1N1 (swine flu) pandemic that infected millions of people worldwide. Antigenic shift is also why healthcare providers keep a close eye on the currently occurring rare cases of A/H5N1 bird flu.

Antigenic drift is the other way flu evolves. In antigenic drift, the flu viruses make tiny changes (genetic mutations) from year to year.

“Antigenic drift occurs as a minor genetic mutation in either influenza A or B, resulting in possible reinfections year after year,” Dr. Mossad explains. “These mutations account for the annual influenza epidemics that make 10 percent to 20 percent of the population ill.”

What do influenza A and influenza B have in common?

Those differences aside, influenza A and B have important things in common:

• They spread the same way and cause similar symptoms.
• Both types are also more likely to affect babies and older adults, causing more serious symptoms in these age groups.
• Healthcare providers treat both types of flu the same way.

Flu causes

An infection of the flu occurs when you come into contact with the flu virus. That can happen if you:

• Are near someone with the flu who coughs, sneezes or even talks to you (droplets moving through the air can get into your nose or mouth; those droplets then make their way to your lungs)
• Touch a contaminated surface and then touch your face, nose, mouth or eyes (the virus can live for up to 48 hours on hard surfaces like counters and desks, phones and computer keyboards)
• Touch the contaminated hands or face of someone with the flu and then touch your face, nose, mouth or eyes

Who’s at risk for influenza A and influenza B?

Anyone can catch these viruses. But babies and older adults are more likely to have more severe symptoms, Dr. Mossad shares.

“Younger babies (age 6 months and younger) are more likely to get infected and more likely to have severe disease caused by influenza than older children,” he notes.

Here’s why: Every time germs like a flu virus invades your kid’s body, their immune system adds details about the virus to its invader database and develops antibodies to protect against future invasions.

Older kids are more likely to have run into more flu viruses than babies, so their immune systems are better trained to fend off these viruses. That means they’ll likely have less serious flu symptoms than babies. And older children often get the flu shot during their annual check-up with their pediatrician. The vaccine isn’t approved for babies 6 months and younger.

Influenza A and influenza B can cause serious complications in people aged 65 or older. They may have more severe flu symptoms or develop a serious illness like pneumonia or myocarditis (inflammation of the heart muscle).

The risk for severe influenza-related illness is higher in older people because they’re more likely to have chronic diseases that worsen with flu, Dr. Mossad clarifies. The CDC reports 9 of 10 people hospitalized for complications of flu had an underlying condition, like chronic obstructive pulmonary disease (COPD).

In some cases, medication to treat a chronic disease increases the risk of having severe complications of flu. For example, Dr. Mossad says that someone who has rheumatoid arthritis (RA) may need to take an immunosuppressant. This medication eases painful RA symptoms. But it also makes it harder for your immune system to fight off flu.

Flu symptoms

In general, you feel lousy whether you have influenza A or influenza B. Unlike the common cold that can creep up on you, flu symptoms hit right away, and they hit hard. Both influenza A and influenza B typically kick off with a fever. Next, comes any combination of symptoms like:

• Headache
• Cough
• Chills
• Diarrhea or vomiting (usually only in kids)
• Fatigue
• Muscle aches
• Runny or stuffy nose (congestion)
• Sore throat

Treatments and when to call the doctor

“The sudden onset of fever, cough and a headache are reasons to call your doctor,” Dr. Mossad advises. And again, he emphasizes that people with underlying chronic medical conditions or are immunocompromised as a result of certain diseases or medications should seek care early even if they have relatively mild symptoms.

“Treatment reduces symptoms and shortens illness when started early, so why stay miserable?” he poses. Another benefit of early treatment is preventing the spread of infection to contacts at risk for severe flu-related illness.

Dr. Mossad typically does nasal or throat flu tests to confirm the diagnosis. Your provider can prescribe antiviral drugs that have U.S. Food and Drug Administration (FDA) approval to treat influenza early (one or two days within the onset of symptoms ):

• Oseltamivir phosphate (Tamiflu®). This medication comes in liquid or pill form. It’s FDA-approved for people 2 weeks of age and older.
• Zanamivir (Relenza®). This medication is a powder that you inhale through your mouth using an inhaler device. It has FDA approval for children aged 7 and older and adults.
• Peramivir (Rapivab®). This is medication that a provider gives people who are in the hospital and are critically ill. You receive the medication via an intravenous line. It’s treatment for babies aged 6 months and older, children and adults.
• Baloxavir marboxil (Xofluza®). This is a pill that you take as a single dose. It’s treatment for children aged 5 to 12 who don’t have chronic medical conditions, and all people aged 12 and older.

What to do if you have the flu

Regardless of influenza type, Dr, Mossad says that the most important things are taking care of yourself and doing your best to avoid spreading the virus.

If the flu bug bites, taking over-the-counter medications like acetaminophen will help with body aches, headache and fever. Here are some other suggestions:

• Get your rest. Your body works hard when you’re sick with the flu. This isn’t the time to power through your day. Rest when you need to, not just when you can.
• Eat well. You can’t go wrong with a cup of chicken soup. But foods that boost your immune system may best help your body fight off flu.
• Drink up. Fever from flu can cause dehydration. Warm water with lemon, herbal tea or a cup of broth can help keep you hydrated.
• Try heat. Heat can ease aching muscles. Try taking a warm bath or shower or using a heat wrap.
• Stay home. Flu is highly contagious. And you’re most contagious the first three or four days after you develop symptoms.

That last suggestion is very important. You may think you can tough it out and go about your day. Again, resist that urge, especially if your daily routine means you’re frequently around many other people.

“In particular, you should stay away from other people who may be at risk for severe illness from the flu,” Dr. Mossad emphasizes. That includes babies, people who are pregnant, people aged 65 and older and people who have chronic illnesses.

Prevention

You may not be able to avoid encountering the flu. But there are many ways to help reduce your risk of flu-related illness. Getting a flu shot every year tops the list. Other best practices are:

• Stay away from people who have flu-like symptoms. If flu is running rampant at work or school, do your best to avoid close contact with them.
• Strengthen your immune system by eating well, staying active and getting enough sleep
• Wash your hands or use a hand sanitizer frequently.
• Avoid touching your face, eyes and mouth.

There are some viruses that make you immune once you’ve had them. Sadly, influenza A and influenza B aren’t among them. “One can get flu A then flu B (or vice versa) in the same flu season,” Dr. Mossad warns.

The bottom line

Influenza A and influenza B may have lots in common. But the ways in which they’re different can affect how flu affects you. Knowing the type of influenza that you or your child have can help healthcare providers select the medication that’s right for you.

And because experts track the differences in how influenza A and influenza B spread, that can help providers plan ways to manage flu season, like letting people know when the flu types are spiking in their communities.