What Is Influenza?

Influenza is an acute respiratory illness caused by influenza viruses, single-stranded RNA viruses of the family Orthomyxoviridae that infect the upper and lower respiratory tracts of humans and a range of animal hosts. It is one of the most studied infectious diseases in history: its annual epidemic burden and its capacity for pandemic spread have together driven the development of global surveillance infrastructure, predictive modeling methods, and rapid vaccine production platforms. The disease circulates seasonally in temperate climates and year-round in tropical regions, producing illness that ranges from mild upper respiratory symptoms to severe pneumonia, particularly in older adults, young children, and immunocompromised individuals.

Virology and Antigenic Variation

Influenza viruses are classified into types A, B, C, and D, with types A and B responsible for seasonal human epidemics. Type A viruses are further subtyped by their surface glycoproteins: hemagglutinin (H) and neuraminidase (N), which mediate host cell attachment and progeny virus release, respectively. The principal mechanism by which influenza evades immune memory is antigenic drift, the gradual accumulation of point mutations in the hemagglutinin and neuraminidase genes under immune selection pressure, producing variants that are no longer fully recognized by antibodies induced by prior infection or vaccination. A more abrupt mechanism is antigenic shift, occurring when two influenza A strains co-infect the same cell and exchange gene segments through reassortment. Shift can produce a novel subtype to which the human population has little pre-existing immunity, the precondition for pandemic emergence. The 1918 H1N1, 1957 H2N2, 1968 H3N2, and 2009 H1N1 pandemic strains each resulted from distinct reassortment events.

Epidemiology and Surveillance

Global monitoring of influenza is coordinated through the WHO's Global Influenza Surveillance and Response System (GISRS), established in 1952, which links 148 National Influenza Centres and seven WHO Collaborating Centers in a year-round network that characterizes circulating strains, detects emerging variants, and informs the biannual selection of strains for seasonal vaccine formulation. In the United States, the CDC's U.S. Influenza Surveillance system integrates data from approximately 400 virology laboratories, 4,000 outpatient sentinel providers, FluSurv-NET hospital networks, and vital statistics systems, producing weekly FluView reports that track geographic spread, subtype distribution, and disease severity. Epidemic intensity is quantified through the influenza-like illness percentage among outpatient visits, excess mortality above seasonal baseline, and hospitalization rates stratified by age group. Mathematical models using the SEIR framework partition the population into susceptible, exposed, infectious, and recovered compartments and are fitted to surveillance data to estimate reproduction numbers and forecast peak timing.

Vaccine Development and Pandemic Preparedness

Annual influenza vaccination remains the primary preventive intervention, but the speed of antigenic drift requires vaccine composition to be updated every season. The WHO's vaccine composition meetings, held in February and September, select the H and N antigens most likely to match the strains circulating in the coming season. Traditional egg-based manufacturing takes four to six months from strain selection to distributed product; cell-based and recombinant platforms reduce this lag. For pandemic preparedness, prototype vaccines against high-consequence subtypes including H5N1 and H7N9 are maintained in advance. The 2009 pandemic demonstrated that a novel H1N1 strain can spread globally within weeks, underscoring the importance of rapid response infrastructure. Research on international influenza surveillance systems and their role in pandemic response reinforces how shared data and laboratory networks form the backbone of both seasonal and pandemic preparedness. Whole-genome sequencing, now applied at WHO's Global Influenza Surveillance and Response System, has become a routine component of strain characterization, enabling near-real-time tracking of emerging mutations.

Applications

Influenza research has applications in a range of fields, including:

• Seasonal vaccine formulation and pandemic countermeasure development
• Mathematical modeling for epidemic forecasting and intervention planning
• Biosensor design for rapid influenza A/B antigen detection
• Hospital capacity planning and resource allocation during peak season
• Animal influenza surveillance at the human-animal interface for zoonotic risk assessment