What is Influenza A H3?
Influenza A H3 (specifically H3N2) is a subtype of influenza A virus classified by the hemagglutinin (H3) surface antigen, which has circulated in humans since the 1968 Hong Kong pandemic and remains one of the two currently circulating influenza A subtypes responsible for seasonal epidemics. 1
Viral Classification and Structure
Influenza A viruses are categorized into subtypes based on two critical surface antigens: hemagglutinin (H) and neuraminidase (N). 1
Three hemagglutinin subtypes (H1, H2, H3) and two neuraminidase subtypes (N1, N2) have caused widespread human disease historically. 1
Currently, only H1N1 and H3N2 subtypes circulate seasonally in humans, with H3N2 being the "H3" virus in question. 2
The H3N2 subtype emerged in 1968 during the Hong Kong flu pandemic and has circulated continuously since then. 3
Evolutionary Characteristics
H3N2 viruses undergo rapid antigenic drift—frequent point mutations in surface proteins—making them the primary driver of severe seasonal influenza epidemics. 2, 4, 5
Since 1968, H3N2 viruses have evolved extensively by adding numerous N-linked glycans to hemagglutinin, increasing the net charge of the HA molecule, and altering receptor binding preferences to escape host immunity. 5
H3N2 viruses pre-dominated during 3 of the last 5 severe influenza seasons, and during the 2016-2017 season, vaccine efficacy against H3N2 was only 28-42% due to rapid antigenic evolution. 5
Increased HA and NA epitope distance between seasons correlates with larger, more intense H3N2 epidemics, higher transmission rates, greater subtype dominance, and a higher proportion of adult cases. 6
Clinical Significance
H3N2 infections are clinically indistinguishable from other influenza types, presenting with abrupt onset of fever, myalgia, sore throat, nonproductive cough, headache, and severe malaise lasting several days. 1
H3N2 is associated with more severe influenza seasons compared to H1N1, with higher rates of hospitalization and complications, particularly in elderly persons and those with chronic medical conditions. 1, 5
During major H3N2 epidemics, hospitalization rates for high-risk persons increase 2- to 5-fold, with approximately 80-90% of excess deaths occurring in persons ≥65 years of age. 1
Transmission and Infectious Period
H3N2 spreads primarily through respiratory droplets from coughing and sneezing of infected persons. 4
The incubation period is 1-4 days (average 2 days), with adults infectious from one day before symptom onset through approximately 5 days after illness onset. 2, 4
Children may remain infectious for more than 10 days, and severely immunocompromised persons can shed virus for weeks or months. 4
Immunity and Vaccine Considerations
Antibodies generated against one influenza A subtype (e.g., H1N1) provide limited or no protection against H3N2 due to distinct hemagglutinin antigens. 2, 4
Even within the H3 subtype, antibodies to one antigenic variant may not protect against newly emerging variants due to continuous antigenic drift. 2, 4
Annual influenza vaccination remains the cornerstone for prevention, with seasonal vaccines including both H1N1 and H3N2 components updated yearly based on circulating strains. 2, 3
For the 2024-2025 season, the H3N2 vaccine component was updated to A/Thailand/8/2022-like (egg-based) or A/Massachusetts/18/2022-like (cell-based/recombinant). 2
Common Pitfalls
Do not assume that rapid influenza diagnostic tests can reliably differentiate H3N2 from other subtypes—definitive subtyping requires RT-PCR or viral culture. 3
Do not underestimate H3N2's capacity for rapid evolution—vaccine mismatch is more common with H3N2 than H1N1, necessitating vigilant strain surveillance. 5
Recognize that H3N2 epidemics disproportionately affect adults and elderly populations compared to H1N1, which tends to impact younger age groups more severely. 6