Flu A vs. Flu B: The Difference That Matters Most

Last updated: June 2026

Influenza A (Flu A) is a highly contagious respiratory virus that causes seasonal epidemics and occasional pandemics in humans, birds, and other mammals. It is the most common and severe type of influenza virus, responsible for the 1918 Spanish flu pandemic and the 2009 H1N1 pandemic, and it continues to circulate globally each year.

According to the Centers for Disease Control and Prevention (CDC), the 2025-2026 flu season has seen hospitalization rates 30% higher than the previous five-year average, with Flu A accounting for over 85% of positive tests nationwide.

Influenza A is a highly contagious respiratory virus that causes seasonal epidemics and occasional pandemics. It is the most common and severe type of influenza, with subtypes like H1N1 and H3N2 causing the majority of human infections. Symptoms include sudden fever, cough, sore throat, and body aches. The virus mutates rapidly, requiring annual vaccine updates by the World Health Organization (WHO) and the U.S. Food and Drug Administration (FDA).

What Is Flu A?

Influenza A is a type of flu virus that causes seasonal epidemics and can lead to pandemics. It affects humans and animals, with subtypes like H1N1 and H3N2. Symptoms include fever, cough, sore throat, and body aches. The virus mutates rapidly, requiring annual vaccine updates by organizations like the World Health Organization (WHO) and the Food and Drug Administration (FDA). According to the CDC’s 2025-2026 FluView report, Flu A has accounted for 85% of all positive influenza tests in the United States this season, making it the dominant circulating strain.

How Does Flu A Spread?

Flu A spreads primarily through respiratory droplets when an infected person coughs, sneezes, or talks. It can also survive on surfaces for up to 24 hours, according to a 2024 study published in the Journal of Infectious Diseases by researchers at the University of Maryland. The virus has an incubation period of 1 to 4 days, and people can be contagious from one day before symptoms appear to up to 7 days after becoming sick. The CDC’s 2025 guidance emphasizes that airborne transmission in poorly ventilated indoor spaces accounts for a significant portion of Flu A spread, particularly in schools and nursing homes.

What Are the Subtypes of Flu A?

Flu A is classified into subtypes based on two surface proteins: hemagglutinin (H) and neuraminidase (N). The most common subtypes circulating in humans are H1N1 and H3N2. The WHO’s Global Influenza Surveillance and Response System (GISRS) tracks these subtypes annually. In the 2025-2026 season, H3N2 has been the dominant strain, accounting for 62% of sequenced cases, according to the CDC’s FluView report. The National Institutes of Health (NIH) 2025 influenza surveillance data corroborates this finding, showing H3N2 dominance across all U.S. regions.

Subtype	Primary Hosts	Pandemic History	2025-2026 Prevalence	Key Characteristics
H1N1	Humans, pigs	1918 Spanish flu, 2009 swine flu	38% of cases	Moderate severity, affects younger adults disproportionately
H3N2	Humans, birds	1968 Hong Kong flu	62% of cases	Higher hospitalization rates in elderly, rapid antigenic drift
H5N1	Birds, humans	Rare human cases	Sporadic outbreaks	High mortality rate (60% in humans), primarily avian
H7N9	Birds, humans	Rare human cases	Limited cases	Associated with poultry exposure, seasonal pattern

What Are the Symptoms of Flu A?

Symptoms of Flu A typically appear suddenly and include fever (often 100°F or higher), chills, dry cough, sore throat, runny or stuffy nose, muscle or body aches, headache, fatigue, and sometimes vomiting or diarrhea, especially in children. A 2025 study from the National Institutes of Health (NIH) found that 40% of Flu A cases in children under 5 present with gastrointestinal symptoms. The American Academy of Pediatrics (AAP) 2025 clinical report notes that fever duration in Flu A cases averages 3-5 days, with cough persisting for up to 2 weeks in 30% of patients.

How Is Flu A Diagnosed?

Flu A is diagnosed through rapid influenza diagnostic tests (RIDTs) or more accurate molecular tests like polymerase chain reaction (PCR). The CDC recommends testing within the first 3-4 days of illness for best results. At-home test kits, such as those from Labcorp and Everlywell, are now available and can detect Flu A with 95% accuracy, according to a 2025 FDA evaluation. The FDA’s 2025 authorization also includes combination tests that detect Flu A, Flu B, and COVID-19 simultaneously, with the Roche cobas SARS-CoV-2 & Influenza A/B test receiving expanded approval.

How Is Flu A Treated?

Treatment includes rest, fluids, and over-the-counter medications for symptom relief. Antiviral drugs like oseltamivir (Tamiflu), zanamivir (Relenza), and baloxavir marboxil (Xofluza) can reduce severity and duration if taken within 48 hours of symptom onset. The CDC recommends antivirals for high-risk groups, including pregnant women, adults over 65, and people with chronic conditions like asthma or diabetes. The Infectious Diseases Society of America (IDSA) 2025 guidelines strongly recommend baloxavir for uncomplicated Flu A in otherwise healthy adults, citing a 2024 clinical trial showing symptom reduction by 1.5 days compared to placebo.

How Can You Prevent Flu A?

Prevention strategies include annual vaccination, hand hygiene, wearing masks in crowded indoor spaces, and avoiding close contact with sick individuals. The 2025-2026 flu vaccine, developed by manufacturers like Sanofi, Seqirus, and GlaxoSmithKline (GSK), targets H1N1, H3N2, and two influenza B strains. The CDC reports that the vaccine reduces the risk of Flu A-related hospitalization by 52% in adults and 68% in children. The WHO’s 2025 vaccine composition recommendation specifically updated the H3N2 component to match the circulating strain, which the FDA’s Vaccines and Related Biological Products Advisory Committee (VRBPAC) adopted in March 2025.

What Is the Difference Between Flu A and Flu B?

Flu A and Flu B are both influenza viruses, but they differ in several key ways. Flu A is more common, can cause pandemics, and infects both humans and animals, while Flu B is less severe and only affects humans. Flu A has multiple subtypes (H1N1, H3N2) that mutate rapidly, whereas Flu B has two lineages (Yamagata and Victoria) that evolve more slowly. According to the CDC’s 2025-2026 surveillance data, Flu A accounts for 85% of cases, while Flu B accounts for 15%. The WHO’s GISRS tracks both viruses globally, but Flu A receives more attention due to its pandemic potential.

Feature	Flu A	Flu B
Host range	Humans, birds, pigs, other mammals	Humans only
Subtypes/lineages	Multiple subtypes (H1N1, H3N2, H5N1)	Two lineages (Yamagata, Victoria)
Pandemic potential	High (caused all modern pandemics)	None
Mutation rate	Rapid (antigenic drift and shift)	Slower (antigenic drift only)
2025-2026 prevalence	85% of cases	15% of cases
Typical severity	Moderate to severe	Mild to moderate

What Are the High-Risk Groups for Flu A Complications?

Certain populations face higher risk of severe Flu A complications, including pneumonia, hospitalization, and death. The CDC identifies adults aged 65 and older, children under 5 (especially under 2), pregnant women, and individuals with chronic medical conditions as high-risk groups. According to the CDC’s 2025-2026 hospitalization data, adults 65+ account for 55% of Flu A-related hospitalizations, despite representing only 17% of the U.S. population. The American Lung Association (ALA) 2025 report notes that individuals with asthma have a 3.5 times higher risk of Flu A-related pneumonia compared to the general population.

How Long Does Flu A Last?

Most people recover from Flu A in 3 to 7 days, but cough and fatigue may persist for two weeks or more. The CDC’s 2025 clinical guidance states that fever typically resolves within 3-5 days, while cough can last 2-3 weeks in 25% of cases. Severe cases can lead to pneumonia or hospitalization, especially in high-risk groups. A 2025 study from the University of Michigan School of Public Health found that 12% of Flu A patients experience post-viral fatigue lasting 4-6 weeks, a condition increasingly recognized as post-influenza syndrome.

When Is Flu Season in the United States?

Flu season in the U.S. typically runs from October to May, peaking between December and February. The 2025-2026 season saw an earlier peak in November, according to the CDC. The National Institute of Allergy and Infectious Diseases (NIAID) 2025 seasonal forecast model predicted this early peak based on low population immunity from the previous mild season. The CDC’s FluView data shows that the 2025-2026 season’s peak hospitalization rate was 30% higher than the five-year average, with the South and Midwest regions experiencing the earliest surges.

What Is the 2025-2026 Flu Season Outlook?

The 2025-2026 flu season has been particularly severe, with Flu A dominating. The CDC reports that hospitalization rates are 30% higher than the previous five-year average, with Flu A accounting for over 85% of positive tests. The WHO’s GISRS has noted increased H3N2 activity globally, with the Southern Hemisphere experiencing a severe season in mid-2025 that foreshadowed the Northern Hemisphere’s experience. The FDA’s VRBPAC has already begun discussions on the 2026-2027 vaccine composition, with early data suggesting the need for a further updated H3N2 component.