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Key takeaways
Influenza A can infect both humans and animals and is often associated with seasonal epidemics and global pandemics due to its ability to change and form various subtypes. Influenza B is slower to mutate, is typically found only in humans, can sometimes lead to more severe cases, especially in children.
Transmission of both Influenza A and B primarily occurs through contact with infected droplets from coughs, sneezes, or talks, with Influenza A also having a rare possibility of animal to human transmission.
Diagnosis for both types involves physical exams and may include tests such as Rapid Influenza Diagnostic Tests (RIDT), with more detailed testing available for more complex or severe cases to differentiate subtypes and inform treatment strategies.
Treatment focuses on supportive care, including rest, fluids, and over-the-counter pain relievers. You can also consult your healthcare provider about antiviral medications, such as Tamiflu, that can shorten the duration of symptoms and reduce the risk of complications, especially important for high-risk groups.
Influenza A vs. B causes | Prevalence | Symptoms | Diagnosis | Treatments | Risk factors | Prevention | When to see a doctor | FAQs | Resources
It starts small. You might wake up with an annoying throat tickle and a runny nose, or you might feel a little more groggy throughout the day than usual. But there’s more on the way. You come down with a fever, chills, body aches, and you’re couch-ridden with the flu.
“The flu” has become a broad term that people use to incorrectly describe a wide range of illnesses. We often hear people say “Oh, I came down with the stomach flu last week,” or “The kids got the 24-hour flu.” But “flu” refers to four types of influenza viruses (A, B, C, and D), most prominently influenza A and influenza B.
Influenza A can infect humans and animals. In most cases, it’s associated with seasonal epidemics in the United States (a.k.a “flu season“) and global pandemics. It’s always changing, so it has various subtypes, including the infamous bird flu (avian influenza) and swine flu. On the other hand, influenza B has two subtypes (Victoria and Yamagata), which occur, for the most part, only in humans and mutate slower, so it’s not really a pandemic risk.
Read on for a complete guide to these two types of flu viruses.
Causes
Influenza A
The most common form of transmission is through tiny droplets created when an infected person talks, sneezes, coughs, or breathes heavily. Type A is also (albeit very rarely) contracted via contact with an infected animal, like a bird or pig. Influenza can also be transmitted through inanimate objects if a sick individual contaminates it, such as a doorknob.
Influenza B
Like influenza A viruses, influenza type B is primarily transmitted through contact with the droplets when an infected individual coughs, sneezes, or talks. Animals aren’t usually susceptible to the influenza B virus, so generally aren’t considered to be carriers.
Influenza A vs. B causes |
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| Influenza A | Influenza B |
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RELATED: Is the flu airborne? Learn how the flu spreads
Prevalence
Influenza A
Influenza A is the most common type of flu. It accounts for approximately 75% of total flu virus infections, and it’s the most likely cause of the “seasonal flu” that hits the U.S. every winter. That’s not a small number, especially considering the 25 to 50 million cases nationwide every year.
During the 2018-19 flu season,, the Centers for Disease Control and Prevention (CDC) tested 1,145,555 specimens for influenza, and of the 177,039 positive results, 95% were influenza A.
Influenza A is further broken down into subtypes based on 2 proteins on the surface of the virus called hemagglutinin and neuraminidase.. The subtypes of both of these proteins result in many different possible combinations and unique influenza A viruses. In addition, small genetic mutations that cause changes in these surface proteins over time may allow these strains to improve their ability to infect people each season. This characteristic presents challenges in predicting a predominantly circulating influenza A virus when it comes to deciding on the composition of the influenza vaccine months before the actual flu season hits. All of these factors together contribute to the predominance of influenza A virus infection each season.
Influenza B
Of course, numbers and percentages can vary from season to season. For example, the early stages of the 2019-2020 influenza season saw influenza B as the most common type, especially among children.
But most years, it takes a backseat to type A. It doesn’t spread quite as easily because it mutates slower and only has two main subtypes: Victoria and Yamagata.. On average, though, type B infections account for around 25% of total flu cases.
Influenza A vs. B prevalence |
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| Influenza A | Influenza B |
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Symptoms
Influenza A
Symptoms of influenza A tend to be similar regardless of subtype. The most common are runny nose, sore throat, fever, chills, body aches, and fatigue.
The main difference is their severity. Type A symptoms often come on stronger and sometimes result in hospitalization or even death.
Influenza B
Type B causes similar symptoms to the ones listed above, but they’re usually milder. However, it still has the potential to increase in severity, causing hospitalization and death, particularly in children.
Influenza A vs. B symptoms |
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| Influenza A | Influenza B |
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(Symptoms can be less severe than influenza A) |
RELATED: Coronavirus (COVID-19) vs. the flu vs. a cold
Diagnosis
Influenza A
A physical exam is the first step. If the doctor identifies common flu signs and symptoms and there is circulating flu activity in the local community, they will likely order a test to confirm the diagnosis. Every flu test requires a healthcare provider to swab a patient’s nose or sometimes throat.
The fastest and most common test is a rapid influenza diagnostic test (RIDT). Results take 10 to 15 minutes, but they might be less accurate than other tests. Plus, RIDTs don’t provide information about the subtypes of influenza A.
Rapid molecular assays are also common in-office tests. They take slightly longer but are more accurate than some RIDTs in that there is a less chance of a test producing a false negative or false positive.
If a doctor needs more detailed information about the virus’ genetic material and strain, they might send the swab to a lab for more in-depth molecular assays that can differentiate influenza A subtypes.
Novel type A viruses, typically animal-borne, don’t often show up on more basic, commercially-available tests. If a doctor suspects a novel virus, they should discuss the possibility of a reverse transcription-polymerase chain reaction (RT-PCR) test with local and state health departments. Viral culture is also another available test which is generally not used for clinical decision-making, but for more extensive evaluation of viruses. Culture is most often used for monitoring of potentially novel influenza A or B viruses that might be considered for the next flu season vaccines.
Influenza B
Like type A, the diagnosis begins with a physical examination, which can sometimes be enough to make a diagnosis. But a test is often necessary for confirmation.
Even though type B is often a less complex virus, RIDTs are less sensitive to its antigens, so these tests aren’t always accurate. Consequently, a doctor might order a more robust test if they suspect a type B infection.
Influenza A vs. B diagnosis |
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| Influenza A | Influenza B |
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Treatments
Influenza A
Supportive care is important in the management of a flu infection. Most people simply hunker down with home remedies like lots of fluids, plenty of rest, and that all-healing homemade chicken soup. Pain relievers like ibuprofen (Motrin) and acetaminophen (Tylenol) are highly effective at mitigating flu symptoms but are not active against the virus.
Antiviral medications are available, but even these are unable to completely eradicate the influenza virus. They are able to shorten the severity and duration of symptoms by approximately one day, therefore prompt initiation should be considered for those who are part of high-risk groups for complications (children, the elderly, other medical conditions), or who have severe symptoms. The rationale to take antiviral medications despite their inability to eradicate viral replication includes that they may reduce risk of complications, including hospitalization. For patients already hospitalized, some data supports initiation of these medications can reduce the risk of death. These medications should be initiated as early in the viral course as possible–ideally within two days of exhibiting flu symptoms. Tamiflu (oseltamivir phosphate), Relenza (zanamivir), Rapivab (peramivir) and Xofluza (baloxavir) are all available antivirals to treat flu. Each of these medications have their own role in therapy. For example, Tamiflu (oseltamivir) is FDA approved for use in adults and pediatric patients as young as 14 days old for treatment of influenza infection for typically a 5 day course; it also is FDA approved for prophylaxis in high risk individuals following a known exposure. Rapivab (peramivir) is available as an intravenously administered medication and is therefore used almost exclusively in patients who are hospitalized. Rapivab (peramivir) can be administered as a single dose as treatment in uncomplicated cases, but with other available options is more likely to be reserved for patients who are hospitalized and unable to receive any of the other treatment options, at which time it is administered as a daily infusion for up to 5 to 10 days. Relenza (zanamivir) is a powder that is inhaled using an inhaler device twice daily for 5 days, and while it can be used in children as young as 7 years of age it should be avoided in people with breathing problems (like asthma or COPD). Relenza (zanamivir) can also be given for post-exposure prophylaxis like Tamiflu (oseltamivir). Xofluza (baloxavir) is a pill taken as a single dose by mouth for both treatment and post-exposure prophylaxis.
Influenza B
Type B treatments are nearly identical to type A treatments. The most common response is simply to let the illness run its course while consuming fluids, resting, and taking over-the-counter medications.
Because influenza B is generally less severe, it might not require antiviral medication, although healthcare providers might still prescribe them for high-risk individuals.
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RELATED: Influenza treatments and medications
Risk factors
Influenza A
Influenza A is unpleasant for the average person. Still, it can be dangerous for seniors (65 or older), children, pregnant women, people with compromised immune systems, or people with a chronic health condition (like heart disease, kidney disease, or asthma).
Influenza B
Risk factors for type B infections and complications are very similar, although influenza B is more prevalent among children.
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RELATED: Which groups are at high-risk for flu complications?
Prevention
Influenza A
One effective strategy for effective flu prevention (and healthy living in general) is to limit potential exposure. This means washing hands, avoiding extended contact with infected individuals, disinfecting infected surfaces, etc. Anyone who’s already come down with influenza A can help stop its spread by staying home and coughing or sneezing into their elbow.
Beyond that, the most effective line of defense is the influenza vaccine (flu shot).All flu shots will now be quadrivalent, meaning they can protect against four different flu viruses. Historically, the flu vaccine was designed to protect against only three different flu viruses: two influenza A viruses (H1N1 and an H3N2 virus) and one influenza B virus. The quadrivalent vaccines include an influenza B virus from a second lineage to provide even broader protection against circulating seasonal influenza viruses.
Strains of influenza A (H3N2) can mutate quickly, though, so health officials have to anticipate its evolution every year. As a result, the seasonal flu vaccine can be less effective in preventing type A infections if that prediction is off.
Influenza B
Taking the same general precautions (washing hands, avoiding sick individuals, etc.) effectively prevents type B infection and spread. The flu vaccine is typically a safe bet for influenza B, but it might not always be a perfect match for the annual strain.
It’s important to dispel a common myth here. Getting a flu shot will not infect someone with influenza A or B. The vaccines contain dead viruses or a single influenza protein, or in the case of the nasal spray vaccine weakened live virus, none of which are enough to infect a human.
How to prevent influenza A vs. B |
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| Influenza A | Influenza B |
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When to see a doctor for influenza A or B
A majority of people will ride out the flu from home with only mild-to-moderate symptoms. But sometimes it’s better to visit your healthcare provider. Anyone with one or more of the risk factors for complications listed above should consider seeing a professional to make sure it doesn’t evolve into a more severe disease or respiratory infection.
A healthcare provider might also be necessary for individuals with severe or prolonged symptoms or other complications like difficulty breathing, chest pain, sudden dizziness, vomiting, neck stiffness, or loss of consciousness.
Frequently asked questions about Influenza A and B
Which is worse: influenza A or influenza B?
Influenza type A and type B are similar, but type A is overall more prevalent, sometimes more severe, and can cause flu epidemics and pandemics.
Is influenza A virus or bacteria?
Influenza A is a virus, although it may present with similar symptoms of common respiratory bacterial infections, like sinusitis.
How long does Type A flu last?
Symptoms typically last five to seven days, although they can linger for up to two weeks. Getting a pre-emptive flu shot or taking antiviral medications can help shorten the duration.
How contagious are influenza A and B?
Influenza is considered a contagious infectious disease. The reproductive number, or R0 (pronounced “R naught”) is a term used to describe just how contagious an infectious disease may be. For seasonal influenza, the R0 is generally 1 or 2–meaning for every person with influenza, they transmit the infection to 1 or 2 other individuals. For context, measles is reported to have an R0 of 12 to 18, while SARS-COV-2 (the virus responsible for COVID-19) has an R0 of 1.5 to 3.5.
The flu virus can spread from an infected person up to 6 feet away through respiratory droplets made when they cough, sneeze, or talk. Less commonly, the virus can live for a period of time on inanimate objects like doorknobs and be transmitted when picked up by an individual who goes on to touch their mouth, nose, or even eyes.
How long is influenza A and B contagious?
People with the flu are contagious one day before symptoms develop and five to seven days after that.
Does influenza go away on its own?
In most cases, yes. Generally, it will run its course in seven to 10 days. High-risk individuals (children, the elderly, those with chronic medical conditions, etc.) may need to see a healthcare provider to prevent further flu complications.
Resources
- Vaccine effectiveness: How well do the flu vaccines work?, CDC
- Rapid influenza diagnostic tests, CDC
- Hospitalization for influenza A versus B, Pediatrics
- Morbidity and mortality weekly report, CDC
- Influenza update, Pharmacy and Therapeutics
- Estimating the lineage dynamics of human influenza B viruses, PLOS One
- Key facts about canine influenza (dog flu), CDC