What are the differences between influenza A and influenza B?

Differences Between Influenza A and Influenza B

Influenza A and B are distinct virus types that both cause epidemic human disease, but influenza A has pandemic potential due to its ability to undergo major genetic shifts and infect multiple animal species, while influenza B is restricted to humans and evolves more slowly.

Structural and Classification Differences

Influenza A Characteristics

• Influenza A viruses are categorized into subtypes based on two surface antigens: hemagglutinin (H) and neuraminidase (N), with 16 H subtypes and 9 N subtypes identified in nature 1, 2
• Currently, H1N1 and H3N2 subtypes circulate in humans and cause seasonal epidemics 1, 2
• Influenza A has a broad host range including humans, birds, pigs, horses, and other mammals, making it a true zoonotic agent 3, 4

Influenza B Characteristics

• Influenza B viruses are NOT categorized into subtypes but are separated into two distinct genetic lineages: Yamagata and Victoria 1, 2
• Influenza B is restricted to humans with no significant animal reservoir, eliminating pandemic potential 3, 4
• Both lineages co-circulate during most influenza seasons 1

Evolutionary and Genetic Differences

Rate of Antigenic Change

• Influenza A undergoes antigenic drift MORE RAPIDLY than influenza B, resulting from frequent point mutations during viral replication 1, 2
• Influenza B evolves more slowly and undergoes antigenic drift at a reduced rate compared to influenza A 1

Pandemic Potential

• Only influenza A can cause pandemics through "antigenic shift" - major genetic reassortment between different viral strains, particularly when human and avian viruses exchange genetic material 1, 3
• The 2009 H1N1 pandemic exemplifies this capability, where a novel virus emerged with limited pre-existing human immunity 1
• Influenza B cannot cause pandemics due to its restriction to humans and lack of animal reservoirs for genetic reassortment 3

Genetic Incompatibility

• Influenza A and B viruses cannot naturally reassort genetic segments with each other due to incompatible viral packaging signals required for segment incorporation into virions 5
• This packaging signal incompatibility contributes to the evolutionary divergence and speciation of these two virus types 5

Clinical and Epidemiological Similarities

Disease Presentation

• Both influenza A and B cause clinically indistinguishable illness characterized by abrupt onset of fever, myalgia, headache, malaise, nonproductive cough, sore throat, and rhinitis 1, 2
• In children, both types commonly cause otitis media, nausea, and vomiting 1
• Influenza A and B show similar pathogenic potential in humans and co-circulate during seasonal epidemics 1

Transmission and Infectious Period

• Both types spread primarily through respiratory droplets from coughing and sneezing 1, 2, 6
• The incubation period is identical: 1-4 days with an average of 2 days 1, 2, 6
• Adults are infectious from the day before symptoms through approximately 5 days after illness onset for both types 1, 2, 6

Immunity and Cross-Protection

• Antibody against one influenza virus type confers limited or NO protection against the other type 1, 6
• Even within the same type, antibody to one antigenic variant may not protect against a new variant of the same subtype or lineage 1, 6
• This lack of cross-protection necessitates annual vaccine updates to match circulating strains 2, 6

Clinical Pitfalls

• Respiratory illness caused by influenza A versus B cannot be distinguished based on symptoms alone - laboratory confirmation is required for definitive diagnosis 1
• The slower evolution of influenza B does not make it less clinically significant - it still accounts for approximately 25% of the annual influenza disease burden 7
• Vaccine formulation must account for both influenza A subtypes (H1N1 and H3N2) and at least one influenza B lineage, though mismatch between circulating and vaccine B lineages has occurred in 5 of 10 seasons historically 1