Flu A vs Flu B: Symptoms, Which Is Worse, and How Long Each Is Contagious

Your test came back positive for flu, and the result says “influenza A” or “influenza B”. Or a friend says they have “the flu B going around” and you want to know whether that is better or worse news. The letters sound like a grading system, as if A is the serious one and B the mild one. It is not that simple.

This guide explains how flu A and flu B actually differ, whether you can tell them apart by symptoms, which one is worse, how long each is contagious, and why the treatment, the test and the vaccine are the same for both.

Quick Answer

Flu A and flu B are two different influenza viruses that cause the same illness: sudden fever, cough, sore throat, body aches, headache and tiredness. You cannot tell them apart by symptoms; only a test can. Influenza A causes about three out of four flu cases, infects animals as well as people, and is the only type that causes pandemics. Influenza B is almost entirely a human virus, makes up about a quarter of cases, hits school-age children hardest, and tends to peak a few weeks later in the season. Flu B is not “mild flu”: among children hospitalized with flu in Canada, a higher share died with flu B than with flu A. Both are contagious from a day before symptoms to about 5 to 7 days after, both are treated with the same antiviral medicines, ideally within 48 hours, and the yearly flu shot covers both.

In this guide:

• Flu A vs Flu B: The Key Differences
• Symptoms: Can You Tell Them Apart?
• Which Is Worse, Flu A or Flu B?
• How Contagious Is Flu A and Flu B?
• How Flu A and B Are Diagnosed
• Treatment for Flu A and Flu B
• Can You Get Flu A and Flu B in the Same Season?
• Does the Flu Shot Cover A and B?
• When to See a Doctor
• The Verdict

Flu A vs Flu B: The Key Differences

Both are influenza viruses and both cause seasonal flu epidemics every year. The differences are mostly in the virus itself, who it infects and how it behaves across a season, not in how you feel when you have it.

The share and timing figures come from the Global Influenza B Study, which analysed more than 1.8 million flu cases in 31 countries. It found flu B was the dominant type in about one season in seven. The incubation figures come from a pooled analysis of published outbreak data; the CDC’s practical summary for flu in general is that symptoms start about 2 days after infection, with a range of 1 to 4 days.

Symptoms: Can You Tell Them Apart?

No. Flu A and flu B produce the same set of symptoms, which tend to come on suddenly. According to the CDC, flu can cause some or all of these:

• Fever or feeling feverish, with chills (not everyone with flu has a fever)
• Cough and sore throat
• Runny or stuffy nose
• Muscle or body aches and headache
• Fatigue
• Vomiting and diarrhea, more common in children than adults

Research has found small differences at the group level. In a Canadian study of 4,155 children hospitalized with flu over eight seasons, headache, abdominal pain and muscle aches were more often linked with flu B. But those are differences in averages across thousands of children, not signs that tell you which type you have. A doctor cannot reliably say “this is flu B” from symptoms either. If knowing matters, for example for a high-risk person, a test is the only way.

So when people search for the symptoms of flu B, the answer is simply the symptoms of flu.

Which Is Worse, Flu A or Flu B?

For the population as a whole, flu A causes more harm, simply because it causes most of the cases and all the pandemics. Its H3N2 subtype is the hardest on older adults: in surveillance data from 29 countries, H3N2 was the virus most over-represented among people aged 65 and over, and it is also the strain the vaccine protects against least well.

For one sick person, though, flu B can be just as serious, and the evidence for children is striking:

• Deaths in hospitalized children: in the Canadian study of 1,510 children with flu B and 2,645 with seasonal flu A at 12 children’s hospitals, deaths attributed to flu made up 1.1% of the flu B group and 0.4% of the flu A group. After adjusting for age and health, the odds of death were about 2.7 times higher with flu B.
• Healthy older children: among otherwise healthy children hospitalized with flu B, those aged 10 and over had the highest odds of needing intensive care.
• Intensive care: a Turkish study of children admitted to intensive care with flu found no significant difference in severity or complications between flu A and flu B.

The honest answer is that neither type is the “mild” one. Which flu you catch matters less than who you are: age, pregnancy and long-term conditions such as asthma, diabetes or heart disease decide most of the risk, and the treatment is the same.

How Contagious Is Flu A and Flu B?

Both are equally contagious as far as everyday advice goes, and the CDC’s timeline applies to both:

• Before symptoms: you can spread flu from about 1 day before you feel sick.
• Most contagious: the first 3 days of the illness.
• How long: up to 5 to 7 days after becoming sick. Young children and people with weakened immune systems may stay contagious for longer.

When can you go back to work or school? The CDC’s current advice for respiratory viruses, flu included, is to stay home until, for at least 24 hours, your symptoms are getting better overall and you have had no fever without using fever-reducing medicine. For the next 5 days, take extra care around others: wash your hands, cover coughs, and consider a mask, especially around older people, babies or anyone with a long-term illness.

How Flu A and B Are Diagnosed

Most flu tests report A and B separately, which is how you come to know which one you have. How much to trust the result depends on the kind of test:

These are pooled sensitivities from two meta-analyses covering more than 150 studies each, compared against laboratory PCR. All the test types were highly specific, above 98% in the 2017 analysis, so a positive result is very likely right. A negative traditional rapid test is much weaker: it misses roughly half of flu B infections, and it does worse in adults than in children. If you have classic flu symptoms during flu season and a negative rapid test, your doctor may still treat you as having flu.

Treatment for Flu A and Flu B

The treatment does not change with the type. There are four FDA-approved flu antivirals in the US: oseltamivir (Tamiflu), zanamivir (Relenza), peramivir (Rapivab) and baloxavir (Xofluza). According to the CDC, all of them are active against both influenza A and B.

• Timing is what matters most. Antivirals work best when started as soon as possible, ideally within 48 hours of the first symptoms. They can shorten the illness by about a day.
• Who should get them: anyone hospitalized with flu or very sick, and people at higher risk of complications, such as adults 65 and over, young children, pregnant women and people with asthma, chronic lung disease, diabetes or heart disease. Most otherwise healthy people with mild flu do not need them, though a doctor can still prescribe them.
• Does Tamiflu work for flu B? Yes, though there are hints that baloxavir may work faster against it. In a Japanese study of 1,742 children and teenagers, fever with flu B lasted a median of about 20 hours on baloxavir against about 34 to 35 hours on oseltamivir and the other neuraminidase inhibitors; for flu A, the drugs performed similarly. It was an observational study, not a randomized trial, and baloxavir is approved in the US only from age 5.

At home, the care is the same for both types: rest, plenty of fluids (our water intake calculator gives a daily baseline to build on when you are sweating with a fever), and acetaminophen or ibuprofen for fever and aches if they are safe for you. Do not give aspirin to children or teenagers with flu, because of the risk of Reye’s syndrome, a rare but serious illness. Antibiotics do not treat flu, because it is a virus; they are only used if a bacterial complication such as pneumonia develops.

Can You Get Flu A and Flu B in the Same Season?

Yes. Influenza A and B are different viruses, so recovering from one does not protect you against the other, and both can circulate in the same winter. Because flu B often peaks a few weeks after flu A, some people catch A early in the season and B later. Flu viruses also keep changing through small genetic shifts, which the CDC calls antigenic drift; that is an important reason people can get flu many times over their lives, even from the same type.

Does the Flu Shot Cover A and B?

Yes. Every US flu vaccine now contains three strains: an A(H1N1), an A(H3N2) and a B/Victoria virus. Vaccines used to have a fourth strain for the B/Yamagata lineage, but Yamagata viruses have not been detected since March 2020, the World Health Organization recommended dropping them from vaccines in September 2023, and US vaccines became trivalent.

How well the shot works depends on the strain. In a meta-analysis of 56 studies, the vaccine reduced the risk of a doctor’s visit for confirmed flu by:

• 61% against A(H1N1)
• 54% against flu B, and 63% in adults over 60
• 33% against A(H3N2), and 24% in adults over 60

So the vaccine protects reasonably well against flu B. Its weak spot is H3N2, which is part of why H3N2 seasons hit older people hardest. The CDC recommends a flu vaccine every season for everyone aged 6 months and older, with rare exceptions.

When to See a Doctor

Call a doctor early, within the first 2 days, if you are at higher risk of complications: 65 or older, pregnant, caring for a sick child under 5, or living with asthma, lung disease, diabetes, heart disease or a weakened immune system. That is the window in which antivirals help most, whichever type of flu it is.

Get emergency care for an adult with:

• Difficulty breathing or shortness of breath
• Persistent pain or pressure in the chest or abdomen
• Persistent dizziness, confusion or being hard to wake
• Seizures, not urinating, or severe muscle pain, weakness or unsteadiness
• Fever or cough that improve and then come back or get worse
• A long-term condition that is getting worse

Get emergency care for a child with:

• Fast or difficult breathing, ribs pulling in with each breath, or bluish lips or face
• Chest pain, or muscle pain so severe they refuse to walk
• Signs of dehydration: no urine for 8 hours, a dry mouth, no tears when crying
• Not alert or not interacting when awake, or seizures
• A fever above 104°F that does not come down with fever medicine, or any fever in a baby younger than 12 weeks
• Fever or cough that improve and then come back or get worse

The Verdict

Flu A and flu B are different viruses with different habits: A is the more common type, infects animals and causes pandemics, while B infects almost only people, peaks later and hits school-age children hardest. Once you are sick, the differences mostly stop mattering. The symptoms are the same, both are contagious for about a week, both can be serious, and both are treated the same way. If you are at higher risk, the important step is not finding out which letter you have but calling a doctor within 48 hours, when antivirals help most. Rest, fluids and good sleep while you recover, and a flu shot each autumn, cover the rest.