Quick Answer
Flu A and flu B are two different types of influenza virus that cause broadly similar symptoms, including fever, cough, sore throat, body aches, and fatigue. Flu A tends to cause larger, more widespread outbreaks and is associated with pandemics, while flu B typically causes smaller, more localised outbreaks and is generally considered somewhat milder on average, though either type can cause serious illness in some people.
What Sets Flu A and Flu B Apart
| Flu A | Flu B | |
|---|---|---|
| Who it affects | Humans and various animals, including birds and pigs | Primarily humans |
| Genetic variability | Higher, leading to more frequent significant mutations | Lower, changes more gradually over time |
| Outbreak pattern | Can cause larger seasonal outbreaks and occasional pandemics | Typically smaller, more localised outbreaks |
| Typical severity | Can range from mild to severe | Often considered somewhat milder on average, though not always |
| Season most common | Often dominates earlier in flu season | Often becomes more prominent later in the season |
Why Flu A Is Linked to Pandemics
Flu A viruses have a wider range of animal hosts and a higher capacity for significant genetic change, sometimes through a process where two different flu strains combine within a shared host. This ability to change more dramatically is why flu A strains have historically been responsible for major pandemics, while flu B has not been linked to pandemics in the same way.
Do the Symptoms Actually Differ?
Symptoms of flu A and flu B overlap significantly and generally cannot be reliably distinguished by symptoms alone. Both commonly cause fever, chills, cough, sore throat, muscle aches, headache, and fatigue, sometimes alongside gastrointestinal symptoms, particularly in children. A laboratory test, rather than symptoms alone, is the only reliable way to confirm which type of flu someone has.
How Flu Is Diagnosed
Healthcare providers can use rapid influenza diagnostic tests or more detailed laboratory testing to identify whether an illness is caused by flu A, flu B, or another respiratory virus entirely. Testing is not always necessary for otherwise healthy individuals with typical symptoms, but may be recommended for those at higher risk of complications or during outbreak monitoring.
Does the Flu Vaccine Cover Both Types?
Seasonal flu vaccines are typically formulated to protect against both flu A and flu B strains predicted to be most common in a given season, alongside protection against multiple flu A subtypes. Only one flu B lineage, B/Victoria, is currently in circulation worldwide, since the other lineage, B/Yamagata, has not been detected anywhere since 2020. As a result, current vaccines are trivalent, covering two flu A strains and one flu B strain, rather than the quadrivalent formula used in earlier years. Because flu viruses change over time, vaccine formulations are reviewed and updated regularly to match circulating strains as closely as possible.
When to Seek Medical Advice
Most healthy adults recover from flu, whether type A or B, with rest and supportive care at home. However, anyone with severe symptoms, difficulty breathing, symptoms that improve then suddenly worsen, or underlying health conditions that increase their risk of complications should seek medical advice promptly, since either flu type can lead to serious illness in vulnerable individuals.
Frequently Asked Questions
Is flu A worse than flu B?
Flu A is more often associated with larger outbreaks and pandemics, but individual illness severity depends on the person, not simply which flu type they have.
Can you get flu A and flu B at the same time?
It is possible, though uncommon, and would typically be identified through specific laboratory testing rather than assumed from symptoms.
Can you tell flu A from flu B without a test?
Not reliably, since the symptoms of both types overlap significantly and a laboratory test is needed to confirm which type is present.
Does the flu vaccine protect against both flu A and flu B?
Yes, seasonal flu vaccines are generally formulated to offer protection against both flu A and flu B strains expected to circulate that season.
Can animals get flu B like they can flu A?
Flu B is understood to primarily affect humans, while flu A can affect a wider range of animals, which is part of why flu A carries a greater pandemic risk.
This article is for general information only and is not a substitute for professional medical advice. If you are concerned about flu symptoms, particularly in a child, an elderly person, or someone with an underlying health condition, contact a healthcare professional.