Each flu season, millions of people worldwide contract influenza, with about 8% of the U.S. population affected annually1 . Influenza A and B viruses are the primary causes of seasonal flu, and while they share many symptoms, they differ in how they spread, mutate, and impact various age groups2 . Understanding these differences can help in recognizing symptoms, seeking timely care, and preventing complications3 .
Symptoms of Flu A vs. Flu B
Both influenza A and B cause illnesses ranging from mild upper respiratory symptoms to severe disease, including high fever, cough, joint pain, and general malaise4 . Symptoms typically appear suddenly and can include:
- Fever and chills2
- Body aches and fatigue1
- Headache and sore throat2
- Cough and nasal congestion or runny nose4
- Occasionally, gastrointestinal symptoms such as vomiting or diarrhea, especially in children3
The clinical features of influenza A and B overlap significantly, and no single symptom can reliably distinguish between the two5 . Influenza infection severity can vary from subclinical (no symptoms) to severe pneumonia or even death due to respiratory failure6 . Generally, flu A symptoms tend to be more severe and aggressive, especially in adults, while flu B is more common in children and may cause milder symptoms7 8.
Differences
Flu A Is More Common
Influenza A viruses cause the majority of seasonal flu cases worldwide, accounting for approximately 70-80% of infections, while influenza B viruses are responsible for about 20-30% 9102. Children experience higher rates of symptomatic influenza infection than adults, with younger children being more susceptible11 12. Notably, influenza B infections tend to occur in slightly older children compared to influenza A, with median ages differing by viral lineage13 12.
Flu A Can Infect Both Humans and Animals
A key difference between the two viruses lies in their host range. Influenza A viruses infect a broad range of hosts, including humans, birds, pigs, and other mammals14 . This wide host range allows influenza A to undergo frequent genetic reassortment (gene swapping), leading to rapid antigenic changes and the emergence of new strains15 6. In contrast, influenza B viruses primarily infect humans and seals, with no known animal reservoirs contributing to their antigenic variation15 6. Consequently, influenza B viruses evolve more slowly and produce fewer antigenic variants15 10.
Seasonal Patterns
Influenza A and B viruses exhibit distinct epidemiological patterns during flu season. Influenza A typically peaks earlier in the season, often in fall and early winter, while influenza B tends to peak later, usually in late winter or early spring9 161. This staggered timing is observed consistently in temperate regions, where influenza B epidemics follow those of influenza A by several weeks on average9 16.
Disease Severity and Complications
Influenza A infections are generally associated with more severe outcomes and higher mortality rates compared to influenza B17 1819. Flu A tends to cause more aggressive symptoms and complications, including pneumonia and respiratory failure, particularly in adults and high-risk populations7 8. However, influenza B can also cause serious illness, especially in children under five and older adults8 . Both viruses can lead to hospitalization and severe complications, but flu A’s rapid mutation and broader host range contribute to its greater severity17 18.
Flu A is More Likely To Cause Pandemics
Influenza A viruses are unique in their ability to cause pandemics—global outbreaks of flu—due to their capacity for antigenic shift, which involves major genetic changes when animal and human influenza viruses reassort14 3. This phenomenon can produce novel strains against which humans have little to no immunity, leading to widespread illness. Influenza B viruses do not undergo antigenic shift and thus have not been linked to pandemics3 . The 2009 H1N1 pandemic is a notable example of an influenza A virus crossing from pigs to humans and causing a global outbreak3 .
Similarities
Transmission Methods
Both influenza A and B viruses spread primarily through respiratory droplets expelled when an infected person coughs, sneezes, or talks20 6. Transmission can also occur indirectly by touching surfaces contaminated with the virus and then touching the eyes, nose, or mouth20 6. Virus shedding begins approximately 24 hours before symptom onset and continues for about 5 to 7 days, with peak contagiousness in the first 3 days of illness6 14. Asymptomatic individuals can also transmit the virus to others6 14.
Diagnosis
Clinical diagnosis of influenza is often based on symptoms but can be confirmed through laboratory testing21 6. Nasal or throat swabs are collected for rapid antigen detection tests or polymerase chain reaction (PCR) assays21 6. Rapid tests can differentiate between influenza A and B but do not provide subtype or strain details21 6. PCR testing offers sensitive and specific identification of influenza viruses, including subtyping for influenza A21 6.
Prevention
Vaccination remains the primary method to prevent both influenza A and B infections20 62. Annual flu vaccines are formulated to protect against the most common circulating strains of both viruses22 2. Additional preventive measures include frequent handwashing, avoiding close contact with infected individuals, covering coughs and sneezes, and disinfecting surfaces20 62.
Treatment
💡 Did You Know? Flu A tends to be more aggressive and cause more severe symptoms compared to flu B. It's more common in adults. In contrast, flu B is more common in children7 .
Treatment approaches for influenza A and B are similar. Most cases can be managed at home with rest, hydration, and over-the-counter medications to relieve symptoms2 23. Antiviral medications, such as oseltamivir (Tamiflu), zanamivir (Relenza), peramivir (Rapivab), and baloxavir marboxil (Xofluza), may be prescribed, especially for high-risk patients or those with severe symptoms24 223. Early initiation of antivirals, ideally within 48 hours of symptom onset, can reduce illness severity and duration24 2.
“Treatment reduces symptoms and shortens illness when started early, so why stay miserable?”
— Susan Wolf, Clinical Director, OSF OnCall Digital Care1
Can You Have Both Flu A and Flu B?
Coinfection with both influenza A and B viruses is possible but uncommon25 26. Simultaneous infection is rare due to differing circulation patterns of the two viruses during the flu season25 26. Infection with one influenza type does not provide immunity or increase susceptibility to the other, meaning a person can contract flu A and flu B sequentially or even concurrently25 261.
When To Seek Care and Who to Contact
It is important to seek medical care if flu symptoms worsen or complications develop. Contact a healthcare provider if you experience:
- Persistent fever lasting more than 72 hours21 6
- Signs of dehydration such as dry mouth or decreased urination21 6
- Worsening respiratory symptoms like cough or difficulty breathing21 6
- Lack of symptom improvement after several days21 6
- Severe or debilitating symptoms affecting daily activities21 6
- Persistent gastrointestinal symptoms such as vomiting or diarrhea21 6
- Relapse or worsening symptoms after initial recovery21 6
High-risk individuals—including young children, older adults, pregnant people, and those with chronic medical conditions—should seek care early for flu symptoms21 62. Urgent or emergency care is warranted for severe symptoms such as high fever unresponsive to medication, chest pain, difficulty breathing, or confusion21 6.
“Flu A is the most common of the two. It can infect both humans and animals and tends to have more mutations. That means it’s more likely to lead to complications.”
— Susan Wolf, Clinical Director, OSF OnCall Digital Care1
A Quick Review
- Influenza A and B cause similar symptoms but differ in host range, mutation rates, and severity15 6.
- Flu A is more common in adults and tends to cause more severe illness, while flu B is more common in children and generally milder7 8.
- Both viruses spread via respiratory droplets and contaminated surfaces, with contagiousness starting before symptoms appear6 14.
- Vaccination and good hygiene practices are key to prevention20 62.
- Early antiviral treatment can reduce flu severity and complications24 2.
- Seek medical care if symptoms worsen, persist, or if you belong to a high-risk group21 6.
