Influenza

Influenza is an acute respiratory illness caused by influenza viruses, which circulate seasonally and occasionally cause pandemics. Its tendency to change antigenically over time means that immunity is imperfectly durable and that vaccines must be updated, making it a distinctive vaccine-preventable disease (Taubenberger & Morens, 2010).

Definition

Influenza is an acute respiratory infection caused by influenza A or B viruses, producing fever, cough, and systemic symptoms, transmitted by respiratory droplets and characterised by antigenic change that drives recurrent epidemics and periodic pandemics.

Scope

This topic covers influenza as a clinical and public-health entity: the influenza viruses and their antigenic variability, seasonal and pandemic patterns of spread, the basis and measurement of vaccine effectiveness, and why protection differs by virus subtype and season. It is a reference overview and does not provide treatment or prescribing guidance.

Core questions

• Why does influenza recur seasonally and occasionally cause pandemics?
• How do antigenic drift and shift affect immunity and vaccine design?
• How is influenza vaccine effectiveness measured, and why does it vary?
• What distinguishes seasonal influenza from a pandemic?

Key concepts

• Influenza A and B viruses
• Antigenic drift
• Antigenic shift
• Seasonal epidemics and pandemics
• Test-negative design for vaccine effectiveness
• Subtype-specific vaccine effectiveness
• Annual vaccine reformulation

Mechanisms

Influenza viruses infect the respiratory epithelium and provoke an immune response directed largely at their surface proteins. These proteins accumulate gradual mutations (antigenic drift) that erode existing immunity, and influenza A viruses can also undergo abrupt reassortment (antigenic shift) that can introduce a novel virus to which populations have little immunity, the basis of pandemics (Taubenberger & Morens, 2010). Because the circulating viruses change, vaccines are reformulated and protection varies; meta-analyses estimate moderate and subtype-dependent vaccine effectiveness, generally lower against A(H3N2) than against A(H1N1) or B (Osterholm et al., 2012; Belongia et al., 2016).

Clinical relevance

Influenza illustrates how a continuously changing virus shapes both individual illness and the design of immunization programmes, and why vaccine effectiveness is measured and reported each season. This entry is descriptive and educational; it explains the disease and the evidence on prevention and is not a basis for individual diagnosis or treatment.

Epidemiology

Influenza causes recurrent seasonal epidemics and, when a novel virus emerges, pandemics with potentially large mortality (Taubenberger & Morens, 2010). Vaccine effectiveness is commonly estimated using the test-negative design and is moderate overall, varying by season and virus subtype (Osterholm et al., 2012; Belongia et al., 2016).

History

Influenza has caused recorded epidemics and pandemics for centuries, including the 1918 pandemic that remains a reference event in public health. The identification of influenza viruses and of their capacity for antigenic drift and shift explained the disease's recurrent and unpredictable behaviour and established the rationale for annually updated vaccines (Taubenberger & Morens, 2010).

Debates

Why is influenza vaccine effectiveness so variable?

Effectiveness differs by season and virus subtype because of antigenic change and mismatch between vaccine and circulating strains; quantifying this variation and improving on it through better-matched or broadly protective vaccines is an active field.

Key figures

• Jeffery K. Taubenberger
• David M. Morens
• Michael T. Osterholm
• Edward A. Belongia

Seminal works

• taubenberger-2010
• osterholm-2012
• belongia-2016

Frequently asked questions

Why does the influenza vaccine need to be given each year?

Influenza viruses change their surface proteins over time (antigenic drift), eroding existing immunity, and the circulating strains vary from season to season. Vaccines are therefore reformulated and given annually to match the expected viruses.

What is the difference between seasonal influenza and a pandemic?

Seasonal influenza arises from viruses already circulating in the population, against which there is partial immunity. A pandemic occurs when a markedly novel influenza A virus emerges, often through antigenic shift, and spreads widely because population immunity is low.