Difference Between Flu A and Flu B

Short answer

• Influenza A and influenza B are the two virus types that cause seasonal flu epidemics in people. Both can cause the same broad set of symptoms, and symptoms alone do not reliably tell them apart.
• The clearest differences are in biology, not in how a person feels. Influenza A infects humans and many animals and is divided into subtypes. Influenza B infects humans and is divided into lineages, not subtypes.
• Influenza A changes by both antigenic drift and antigenic shift. Influenza B changes by drift only. That difference helps explain why only influenza A is known to have caused pandemics.
• Both types circulate during flu season, the mix changes from season to season and within a season, and laboratory testing, not a symptom checklist, is what distinguishes one type from the other.
• In the United States, current flu vaccines are trivalent. They cover one influenza A(H1N1) virus, one influenza A(H3N2) virus, and one influenza B/Victoria lineage virus.

Why two letters cause so much confusion

A laboratory report reading influenza A or influenza B can look like a clear verdict, as if one letter must mean a mild illness and the other a serious one. In practice, the letters describe which virus was detected, not how ill a person will become. Both types spread from person to person, cause seasonal epidemics, and can cause mild illness, severe illness, and, in some cases, death. The World Health Organization (WHO) describes seasonal influenza as an acute respiratory infection caused by influenza viruses, common in all parts of the world, with most people recovering without treatment and severe illness concentrated mainly among people at higher risk.

The comparison matters at the population level: one type has an animal reservoir and subtypes, the other circulates in people and has lineages, and one can change suddenly while the other changes gradually. Those differences shape surveillance, vaccine composition, and pandemic planning, but they do not let an individual guess their type from symptoms. For a fuller account of influenza B on its own, see the Amberaisle guide What Is Flu B?. That guide explains a single type, while this article explains the comparison itself.

What the letters A and B actually mean

Influenza viruses are grouped into types, and A and B are two of four types, alongside C and D. WHO states that there are four types of influenza viruses, types A, B, C, and D, and that influenza A and B viruses circulate and cause seasonal epidemics of disease. Influenza C is detected less frequently and usually causes mild infections, and influenza D viruses primarily affect cattle and are not known to infect or cause illness in people. When health agencies talk about seasonal flu, they are normally talking about A and B.

The type label is based on the virus itself, not on the patient, the season, or severity. Laboratory tests can distinguish type A from type B readily, while clinical symptoms cannot. That point, made in WHO influenza biology material, is the most useful correction to popular comparisons: lists assigning one symptom set to flu A and another to flu B should not be used to identify a case.

Naming adds another layer of confusion. Influenza A viruses carry names such as A(H1N1) and A(H3N2). The letters H and N refer to two proteins on the surface of the virus, hemagglutinin and neuraminidase. The numbers distinguish different forms of those proteins. Influenza B viruses do not use that system. They are not classified into subtypes. Instead, they are broken down into lineages, named B/Victoria and B/Yamagata. A name such as B/Victoria therefore tells a reader about lineage, while a name such as A(H3N2) tells a reader about subtype. WHO also notes that the currently circulating A(H1N1) virus is written as A(H1N1)pdm09, because it caused the pandemic in 2009 and replaced the previous A(H1N1) virus. The label pdm09 is a historical marker, not a separate type.

Influenza A: the type with an animal reservoir

Influenza A is defined, in practical terms, by breadth. It infects humans and many animals. The United States Centers for Disease Control and Prevention (CDC) lists ducks, chickens, pigs, horses, whales, and seals among the animals in which flu A viruses are found. That breadth matters because it gives influenza A more opportunities to encounter new hosts, to exchange genetic material, and to introduce a virus to people that human immune systems have not met before.

Influenza A viruses are divided into subtypes according to combinations of the hemagglutinin and neuraminidase proteins on the virus surface. WHO states that the subtypes currently circulating in humans are A(H1N1) and A(H3N2). Many other subtypes circulate in birds, so only a small number cause ordinary seasonal flu in humans at any given time. Influenza A is also the type associated with pandemics: WHO states plainly that only influenza type A viruses are known to have caused pandemics. That does not mean every new influenza A virus causes a pandemic, and it does not mean influenza B is harmless. It means the pandemic record, so far, belongs to type A, for reasons connected to host range and to antigenic shift, explained below.

Influenza B: the human type, organized by lineage

Influenza B is narrower in host range and simpler in classification, but it is not a minor footnote to seasonal flu. WHO states that influenza B viruses belong to either the B/Yamagata or the B/Victoria lineage. According to WHO influenza biology material, influenza B viruses only infect humans, so their evolution is tied to circulation in people rather than to a large animal reservoir.

The lineage distinction matters because it has changed recently. CDC states that influenza B/Yamagata viruses have not been detected by global flu surveillance after March 2020, and that the risk of infection with B/Yamagata lineage viruses is considered to be low at this time. CDC also states that it is not known whether influenza B/Yamagata lineage viruses are extinct: absence from surveillance is not proven extinction, and surveillance continues. Reports of B/Yamagata detections after March 2020, where samples were available, were confirmed as B/Victoria lineage viruses or as the B/Yamagata component of live attenuated, meaning nasal spray, vaccines. In short, influenza B has a human host range, a lineage classification, and B/Victoria as the lineage currently represented in United States flu vaccines.

The comparison at a glance

The table below collects only the distinctions supported by the primary sources used for this article. Where a comparison varies by season, the table says so rather than giving a single fixed figure.

Feature	Influenza A	Influenza B
Host range	Humans and many animals, including ducks, chickens, pigs, horses, whales, and seals	Humans
Classification	Subtypes based on hemagglutinin and neuraminidase, for example A(H1N1) and A(H3N2) in people	Lineages, not subtypes: B/Victoria and B/Yamagata
Change over time	Antigenic drift and antigenic shift	Antigenic drift only
Seasonal epidemics in people	Yes	Yes
Pandemics known to date	Yes, only type A is known to have caused pandemics	No pandemic record
Symptoms in an individual	Same broad symptom set as influenza B, symptoms do not reliably distinguish type	Same broad symptom set as influenza A, symptoms do not reliably distinguish type
How type is confirmed	Laboratory testing	Laboratory testing
Share of cases in a season	Varies by season and within a season, no single fixed share	Varies by season and within a season, no single fixed share
Current United States vaccine component	One A(H1N1) virus and one A(H3N2) virus	One B/Victoria lineage virus

Popular articles often give a fixed percentage split between flu A and flu B. Those figures may describe a particular season or week, but they do not hold as a permanent rule. CDC notes that over the course of a flu season, different types and subtypes circulate and cause illness, and its weekly FluView reports show the mix changing week by week. A single season snapshot should never be quoted as the general share for flu A and flu B.

Symptoms: shared, overlapping, and unreliable as a test

Both influenza A and influenza B cause the same broad illness: an acute respiratory infection that usually comes on suddenly. WHO lists sudden onset of fever, cough, usually dry, headache, muscle and joint pain, severe malaise, meaning feeling unwell, sore throat, and runny nose. CDC lists fever or feeling feverish and chills, cough, sore throat, runny or stuffy nose, muscle or body aches, headaches, and fatigue, and notes that some people may have vomiting and diarrhea, though this is more common in children than adults. CDC also stresses that not everyone with flu will have a fever.

That overlap is why this article repeats one point: symptoms alone do not reliably distinguish flu A from flu B. Age, general health, strain, and prior exposure can all influence how an illness feels, and none maps cleanly onto the A or B label. The only reliable distinction in an individual case is laboratory testing, described below.

Two misconceptions deserve correction. First, the absence of fever does not rule out flu, a point CDC makes explicitly for both types. Second, vomiting and diarrhea are not defining flu symptoms: influenza is primarily a respiratory illness, and stomach symptoms alone do not identify flu type. Timing also overlaps: WHO states that symptoms begin 1 to 4 days after infection and usually last around a week, with cough sometimes lasting 2 weeks or more. Those ranges describe influenza in general, not a difference between A and B.

Severity: what can be said, and what cannot

Both influenza A and influenza B can cause mild illness, severe illness, complications, and death. WHO estimates around a billion cases of seasonal influenza annually, including 3 to 5 million cases of severe illness, and 290,000 to 650,000 respiratory deaths annually. Those figures cover seasonal influenza as a whole, not A and B separately, and they underline why a comparison that calls one type mild and the other severe would be misleading.

Severity depends on more than type. WHO lists people at greater risk of severe disease or complications as pregnant women, children under 5 years of age, older people, individuals with chronic medical conditions such as chronic cardiac, pulmonary, renal, metabolic, neurodevelopmental, liver or hematologic diseases, and individuals with immunosuppressive conditions or treatments. CDC similarly lists people 65 years and older, people of any age with certain chronic medical conditions such as asthma, diabetes, or heart disease, pregnant women, and young children as at higher risk of serious flu related complications. Complications listed by CDC include pneumonia, bronchitis, and sinus and ear infections, and flu can also make chronic health problems worse, for example by triggering asthma attacks.

A fair statement about severity is therefore a careful one: in some seasons or age groups, one type may account for more severe outcomes, and in other seasons the pattern may differ. A ranking drawn from one season should not become a permanent label. Both types can be serious, risk is concentrated in the groups listed above, and the type label alone does not settle how serious an individual illness will be.

Antigenic drift and antigenic shift: the key biological difference

Both types change over time, but they do not change in the same ways. Antigenic drift is the gradual process. CDC describes drift as small changes, or mutations, in the genes of influenza viruses that can lead to changes in the surface proteins hemagglutinin and neuraminidase. These small changes happen continually. Over time, antibodies formed against an older strain may recognize a newer strain less well, which is one reason people can get flu more than once and one reason vaccine composition is reassessed each year. Both influenza A and influenza B change by drift, and older CDC material adds that influenza B viruses evolve more slowly than influenza A viruses.

Antigenic shift is the abrupt process, and it applies to influenza A only. CDC describes shift as an abrupt, major change in an influenza A virus, resulting in new hemagglutinin or new hemagglutinin and neuraminidase proteins in influenza viruses that infect humans. When shift happens, most people have little or no protection against the new virus. CDC states that type A viruses undergo both kinds of change, while influenza type B viruses change only by the more gradual process of antigenic drift. WHO makes the same point in different words: occasionally a new subtype of influenza A emerges with a hemagglutinin to which most humans have not previously been exposed, and this is known as antigenic shift, while the surface proteins of type A and B viruses continually change by mutation, a process referred to as antigenic drift.

The 2009 example illustrates shift: WHO notes that A(H1N1)pdm09 caused the pandemic in 2009 and replaced the previous A(H1N1) virus, and CDC describes the spring 2009 emergence of a new H1N1 virus with a new combination of genes, which quickly spread and caused a pandemic. That is why pandemic planning focuses on new influenza A subtypes, while seasonal planning covers both A and B. It is not evidence that influenza B can never cause a large outbreak: influenza B causes seasonal epidemics, and epidemics can be large. The distinction is between seasonal epidemics, which both types cause, and pandemics, which so far only type A is known to have caused.

Seasonality and surveillance: when each type circulates

Seasonal influenza is often described as a winter illness, and in temperate climates that description is broadly correct. WHO states that in temperate climates seasonal epidemics occur mainly during winter, while in tropical regions influenza may occur throughout the year, causing outbreaks more irregularly. A reader in a tropical region should not assume flu follows the same calendar as a reader in the northern United States.

For the United States, CDC states that flu season occurs in the fall and winter, that the peak has occurred anywhere from late November through March, and that it monitors flu viruses in weekly FluView reports from October through May. The health impact of a season varies from year to year, and the mix of types and subtypes changes within the season.

Surveillance makes those statements possible. WHO monitors influenza viruses globally and recommends vaccine compositions twice a year, and CDC characterizes circulating viruses each year to inform vaccine composition. When a news report says flu A is dominant this week, it is reporting that week’s data, not a law of nature. Host range, classification, drift and shift, and pandemic history are stable facts. The weekly mix and the timing of the peak change.

Vaccines: how the A and B difference shows up in a vial

Vaccines are where the A versus B distinction becomes concrete, because a flu vaccine must cover both types. CDC states that all flu vaccines in the United States are trivalent, meaning three component, vaccines since the 2024 to 2025 flu season. Trivalent flu vaccines in the United States include vaccine viruses or viral proteins from one influenza A(H1N1) virus, one influenza A(H3N2) virus, and one influenza B/Victoria lineage virus.

That composition tells the comparison in miniature. Two components are influenza A, because two subtypes circulate in people. One is influenza B, because only one lineage is currently represented. Earlier quadrivalent, or four component, vaccines also included B/Yamagata. CDC explains that the B/Yamagata component was removed beginning with the 2024 to 2025 flu season because influenza B/Yamagata viruses have not been detected by global flu surveillance after March 2020, and because WHO and the United States Food and Drug Administration advisory committee recommended excluding it. CDC also states that if B/Yamagata viruses begin spreading again, CDC, the Food and Drug Administration, and flu vaccine manufacturers will adapt accordingly, since vaccine composition is assessed annually and updated for most seasons.

In general terms, CDC states that everyone ages 6 months and up should get an annual flu vaccine, and WHO states that annual vaccination is recommended, with immunity going away over time, especially for people at high risk and their carers. Those are agency recommendations for populations, not assessments of individual circumstances. They explain why vaccines are updated every year: drift changes the viruses, and last year’s composition may not match this year’s circulating viruses. A vaccine that covers both A and B does not mean a vaccinated person cannot get flu. It means the vaccine is designed against the viruses expected to circulate, and its match to those viruses, together with a person’s immune response, determines how much protection it provides in a given season.

Testing: how the type is actually identified

If symptoms cannot distinguish flu A from flu B, testing can. WHO states that collection of respiratory samples and laboratory testing is required to establish a definitive diagnosis, with laboratory confirmation commonly performed using direct antigen detection, virus isolation, or detection of influenza specific RNA by reverse transcriptase polymerase chain reaction, known as RT-PCR. WHO also notes that rapid diagnostic tests are used in clinical settings but have lower sensitivity compared with RT-PCR, and that their reliability depends largely on the conditions under which they are used.

A laboratory result naming influenza A or influenza B is therefore a statement about the virus detected in the sample, subject to the limits of the test used. Testing also serves surveillance: samples identifying a type, subtype, or lineage contribute, in aggregate, to the picture of which viruses are circulating, including whether B/Yamagata has reappeared. WHO also notes that during periods of low influenza activity, other respiratory viruses can present as influenza like illness, making clinical differentiation difficult.

Can you have flu A and flu B, and can you get flu twice?

Infection with one type does not give lasting protection against the other type. CDC background material states that antibody against one influenza virus type or subtype confers limited or no protection against another type or subtype. A person who has had influenza A is therefore not automatically protected against influenza B, and the reverse is also true.

A person can also get flu more than once. Drift means a newer strain may not be recognized as well by antibodies against an older strain, as CDC explains. A second illness in the same season could involve the other type, a different subtype, or a drifted strain of the same type. This article does not claim any frequency for double infections, because that depends on circulation and testing, but the mechanism is clear: immunity to one virus does not equal immunity to all influenza viruses.

What does not differ, and why that matters

Several important things do not differ. Both types spread in the same broad ways: WHO describes spread through infectious droplets when an infected person coughs or sneezes, and through contaminated hands, while CDC states that people with flu can spread it to others up to about 6 feet away, mainly by droplets, and less often by touching a contaminated surface and then touching their own mouth or nose. Both types cause the same broad symptoms and complications, are covered by United States seasonal flu vaccines, and are monitored by the same surveillance systems.

Prevention guidance, in general terms, is also shared: hand washing, covering coughs and sneezes, staying home when unwell, and avoiding close contact with sick people. Those measures are general public health guidance, not a guarantee against infection, and they apply regardless of which type is circulating. This article is general information only. It does not diagnose any illness, recommend any treatment for any individual, or give personal vaccine advice. Where a figure varies by season, it says so instead of giving a single fixed number. Where surveillance has not detected a virus, it reports the absence of detection rather than claiming extinction.

Frequently asked questions

Is flu A worse than flu B?

There is no single, permanent answer that holds for every season and every group. Both types can cause mild illness, severe illness, complications, and death, and severity depends on the strain, the person’s age and health, prior immunity, and access to care. The type label alone does not predict how ill an individual will become.

Can you tell flu A from flu B by symptoms?

No, not reliably. Both types cause the same broad symptoms, and not everyone with flu has a fever. WHO influenza biology material states that the two types can be readily distinguished by laboratory tests but not by clinical symptoms. A symptom checklist can describe flu in general, but it cannot identify the type.

What is the main difference between flu A and flu B?

The main differences are host range, classification, and change over time. Influenza A infects humans and many animals, is divided into subtypes such as A(H1N1) and A(H3N2) in people, and changes by both drift and shift. Influenza B infects humans, is divided into lineages B/Victoria and B/Yamagata rather than subtypes, and changes by drift only. Only influenza A is known to have caused pandemics.

Is flu B less common than flu A?

The mix varies. Both types cause seasonal epidemics, and the share of each type changes from season to season and within a season, as CDC weekly surveillance shows. A percentage from one season or one week should not be quoted as the general share.

Can you get flu A and flu B at the same time, or one after the other?

Immunity to one type gives limited or no protection against the other type, because the viruses have different surface proteins. A person can therefore have an illness caused by one type and later have an illness caused by the other type. This article does not give a frequency for same season double infections, because that depends on circulation and testing patterns.

Why do flu vaccines cover both flu A and flu B?

Because both types cause seasonal epidemics in people, a vaccine designed for seasonal flu must address both. In the United States, current trivalent vaccines cover one A(H1N1) virus, one A(H3N2) virus, and one B/Victoria lineage virus. The composition is reviewed annually and updated for most seasons.

What happened to the B/Yamagata lineage in vaccines?

CDC states that influenza B/Yamagata viruses have not been detected by global flu surveillance after March 2020, and the B/Yamagata component was removed from United States flu vaccines beginning with the 2024 to 2025 flu season. CDC also states that it is not known whether B/Yamagata viruses are extinct, and that surveillance continues.

Does flu B only affect children?

No. Influenza B infects people of all ages. WHO lists children under 5 years of age among the groups at greater risk of severe disease, alongside pregnant women, older people, and people with chronic medical conditions or immunosuppressive conditions. Risk groups describe who is more likely to develop severe disease, not who can be infected.

How long does flu last, for flu A and for flu B?

WHO states that symptoms begin 1 to 4 days after infection and usually last around a week, with cough sometimes lasting 2 weeks or more. CDC describes recovery in most people within a few days to less than 2 weeks. Those ranges describe influenza in general. Duration alone does not distinguish flu A from flu B.

If I had flu A before, am I protected against flu B?

No, not reliably. Antibody against one influenza virus type or subtype confers limited or no protection against another type or subtype, according to CDC background material. Prior infection with influenza A does not automatically protect against influenza B, and the same applies in reverse.

Sources & references

• World Health Organization, Influenza (seasonal) fact sheet, published 28 February 2025. Used for the four virus types, seasonal epidemics caused by A and B, subtypes A(H1N1) and A(H3N2) in humans, lineages B/Yamagata and B/Victoria, pandemics known only for type A, symptoms and timing, high risk groups, transmission, diagnosis, prevention, vaccine updating, and global surveillance and vaccine composition recommendations.
• World Health Organization influenza biology material. Used for the statement that type A and type B can be readily distinguished by laboratory tests but not by clinical symptoms, that influenza B viruses only infect humans and are not classified into subtypes, that B/Victoria is the only type B virus currently circulating in humans, and for the drift and shift descriptions.
• United States Centers for Disease Control and Prevention, Understanding Flu Viruses. Used for the two main types of human flu viruses, seasonal epidemics, flu A subtypes based on surface protein genes, different types and subtypes circulating over a season, flu A in many animals, pandemic flu as a global outbreak of a new flu A virus, and antigenic characterization and surveillance.
• United States Centers for Disease Control and Prevention, How Flu Viruses Can Change: Drift and Shift, and Understanding Influenza Viruses. Used for drift as small continual changes in hemagglutinin and neuraminidase, shift as an abrupt major change in influenza A only, the 2009 H1N1 example, and the statement that type B viruses change only by drift.
• United States Centers for Disease Control and Prevention, Trivalent Influenza Vaccines, updated 11 May 2026. Used for trivalent vaccines in the United States since the 2024 to 2025 flu season, the three components, the quadrivalent to trivalent transition, B/Yamagata not detected after March 2020, the low current risk assessment, the unknown extinction status, annual review and updating, and the annual vaccine recommendation for ages 6 months and up.
• United States Centers for Disease Control and Prevention, Flu Questions and Answers and Flu Symptoms and Severity material. Used for the symptom list, the point that not everyone with flu has a fever, complications including pneumonia, bronchitis, and sinus and ear infections, worsening of chronic conditions, higher risk groups, United States flu season timing with peak from late November through March, weekly FluView reporting from October through May, spread up to about 6 feet away mainly by droplets, and limited or no cross protection between types or subtypes.