What are the differences between influenza A and influenza B?

Differences Between Influenza A and Influenza B

Influenza A and B both cause clinically indistinguishable seasonal epidemics in humans, but influenza A is categorized into subtypes (H1N1, H3N2) based on surface antigens and has pandemic potential through antigenic shift, while influenza B is divided into two genetic lineages (Yamagata and Victoria) without subtypes and cannot cause pandemics because it lacks an animal reservoir. 1

Viral Classification and Structure

Subtype Categorization:

• Influenza A viruses are classified into subtypes based on two surface antigens: hemagglutinin (H) and neuraminidase (N), with currently circulating human subtypes being H1N1 and H3N2. 2, 1
• Influenza B viruses are not divided into subtypes; instead, they are separated into two distinct genetic lineages—Yamagata and Victoria—that co-circulate during most influenza seasons. 2, 1

Evolutionary Dynamics and Pandemic Potential

Rate of Antigenic Change:

• Influenza A undergoes antigenic drift markedly faster than influenza B due to more frequent point mutations during viral replication, which is the primary driver of seasonal epidemics. 2, 1
• Influenza B evolves more slowly and undergoes antigenic drift at a reduced rate compared to influenza A. 2, 1

Pandemic Capability:

• Only influenza A can cause pandemics through antigenic shift—major genetic reassortment between distinct viral strains, particularly between human and avian viruses—because influenza A has an animal reservoir in aquatic birds. 1, 3
• Influenza B cannot cause pandemics because there is no animal reservoir for this virus type; it is restricted to humans. 3
• The 2009 H1N1 pandemic exemplifies influenza A's pandemic capability, where a novel virus emerged with limited pre-existing population immunity. 2

Clinical Presentation

Symptom Profile:

• Both influenza A and B produce clinically indistinguishable illness characterized by abrupt onset of fever, myalgia, headache, severe malaise, nonproductive cough, sore throat, and rhinitis. 1
• In pediatric patients, both virus types commonly cause otitis media, nausea, and vomiting. 1
• Symptoms alone cannot reliably differentiate influenza A from B, necessitating laboratory confirmation (RT-PCR or rapid antigen testing) for definitive diagnosis. 1

Transmission Characteristics

Spread and Infectious Period:

• Both influenza A and B spread primarily via respiratory droplets expelled during coughing and sneezing. 2, 1
• The incubation period for both viruses is 1–4 days (average approximately 2 days). 2, 1
• Adults are infectious from one day before symptom onset through approximately five days after onset for either virus type. 2, 1
• Children can be infectious for more than 10 days after symptom onset for both types. 2

Immunologic Cross-Protection

Type-Specific Immunity:

• Antibodies generated against one influenza type provide limited or no protection against the other type due to differences in surface antigens. 2, 1
• Even within the same type, antibody to one antigenic variant may not protect against a newly emerging variant of the same subtype or lineage. 2, 1
• Patients who recover from influenza A remain fully susceptible to influenza B infection during the same season. 4

Vaccine Considerations

Vaccine Composition:

• Seasonal influenza vaccines include both circulating A subtypes (H1N1 and H3N2) and at least one B lineage. 1
• Historically, a mismatch between the vaccine B component and the circulating B lineage occurred in 5 of 10 seasons from 2001–2011, because trivalent vaccines contained only one B lineage while both lineages co-circulated. 2
• Quadrivalent vaccines now include both B lineages (Yamagata and Victoria) to address this mismatch problem. 2

Key Clinical Pitfalls

Common Misconceptions:

• Do not assume prior influenza A infection provides any protection against influenza B during the same season; patients require continued infection control measures and may benefit from antiviral prophylaxis if exposed to the other type. 4
• Do not rely on clinical presentation to distinguish between types; laboratory confirmation is essential for accurate diagnosis and epidemiologic surveillance. 1
• Do not withhold vaccination from patients who have already had confirmed influenza during the current season, as the vaccine protects against types not yet encountered. 4