While flu A and flu B share many symptoms, there are differences between how the viruses spread, who they affect most, and the potential severity of the illness. 

Flu A Is More Common

In the United States, about 3-11% of the population gets the flu each year, with an average of 8% during seasons of moderate severity. Flu A is more common, accounting for around 75% of all cases, while flu B causes about 25%. 

Children under 18 are twice as likely as adults to develop a symptomatic flu infection. Flu A is more common in children under 2 years old, while flu B is more common in children under 4.

Flu A Can Infect Both Humans and Animals

Flu A can infect humans and animals, including birds, pigs, dogs, cats, bats, cows, and horses. Its ability to spread between animals and humans allows flu A to change frequently and form new strains. In contrast, flu B only infects humans and does not spread to animals. As a result, it mutates (changes) more slowly and produces fewer new strains.

Seasonal Patterns

Flu A typically appears earlier in the flu season, peaking in the fall and early winter, while flu B tends to show up later. In the northern hemisphere, where flu season runs from October to March, flu A typically peaks around February, while flu B peaks around March. 

Disease Severity and Complications 

While flu A and flu B can cause life-threatening complications like pneumonia, flu A is generally more severe. High-risk groups include older adults, young children, pregnant people, and those with weakened immune systems or chronic conditions.

Flu B can also lead to severe complications, particularly in children under five.

Flu A is More Likely To Cause Pandemics

An epidemic is a disease outbreak affecting many people in a specific region, while a pandemic occurs globally across multiple countries or continents.

Flu A is more likely to cause a pandemic because it can spread from animals to humans, creating new strains. Flu A also mutates faster, leading to more new virus versions. In contrast, flu B only affects humans and mutates five times slower, making pandemics less likely.

Flu A and B share many similarities in how they spread, how providers diagnose them, and how you can manage them.

Transmission Methods

Flu A and B spread in the same way—through respiratory droplets when someone coughs, sneezes, or talks. You can catch the flu when you breathe in droplets from an infected person. You can also get the flu by touching surfaces with the virus and then touching your mouth, nose, or eyes.

People with the flu can spread the virus one day before symptoms start and for 5-7 days afterward. You’re most contagious during the first three days. You can also spread the virus without having symptoms.

Diagnosis 

Providers usually diagnose the flu based on symptoms. However, testing is important for people at higher risk to help guide treatment or public health decisions during outbreaks.

Providers swab your nose or throat to diagnose flu A and flu B.

• Rapid antigen tests: Provide results in 15-30 minutes and can distinguish between flu A and flu B; cannot determine subtypes of flu A, strain information, or seasonal vs. novel flu A
• PCR test: Results can take hours or days and can help confirm rapid test results and distinguish between flu A subtypes and strains

Prevention

Preventing both flu A and flu B involves taking similar steps. Here are some recommendations:

• Get the yearly flu shot by November
• Wash your hands often
• Keep some distance from others while talking when possible
• Try to avoid close contact with sick people and wear a mask in crowded places
• Cover your mouth when you cough or sneeze to help reduce the chance of spreading the flu if you have it

Treatment

Both types of flu typically resolve within a few weeks with supportive care like rest, hydration, home remedies, and over-the-counter (OTC) medications like Tylenol (acetaminophen).

Providers may prescribe antiviral medicines like Tamiflu (oseltamivir), Relenza (zanamivir), or Rapivab (peramivir) for both types of flu. These medications work best when taken within 48 hours of noticing symptoms to help shorten the illness and prevent complications.

You can get both flu A and B. However, it’s rare to get both flu A and flu B at once because the two viruses often don’t circulate in the same place at the same time. Having flu A doesn’t make you more or less likely to catch flu B. 

Reach out to your primary care provider if you:

• Have a fever that lasts more than three days
• Experience mild to moderate dehydration (e.g., dry mouth, reduced urination)
• Experience symptoms that don’t improve after a few days
• Have concerns about preexisting health conditions being affected by the flu
• Need advice on antiviral medications or home care

Go to an urgent care clinic if you or someone you know experiences:

• Persistent high fever—over 103 degrees Fahrenheit (39.4 degrees Celsius) in adults or 102 degrees Fahrenheit (38.9 degrees Celsius) in children
• Severe body aches or fatigue, making it difficult to function
• Vomiting or diarrhea that doesn’t improve
• Symptoms that worsen after initially improving

Seek emergency care if you or someone you know experiences severe or life-threatening symptoms, such as:

• Bluish lips or face
• Chest pain 
• Confusion, fainting, or dizziness
• Difficulty breathing
• Ribs pulling with each breath (in children)
• Seizures 
• Severe dehydration (e.g., no urination)
• Severe weakness
• Temperature above 104 degrees Fahrenheit (40 degrees Celsius)

Flu A and B share similarities but have some key differences. Flu A is more common and infects humans and animals, allowing it to change rapidly and potentially cause pandemics. Flu B affects only humans and is usually milder, though it can still be severe in children, older adults, and people who are pregnant.

Preventing both types of flu includes getting a flu shot, washing your hands, and avoiding people who have the virus. Knowing when to see a healthcare provider can help you get care quickly and avoid complications.