Influenza viruses are among the most significant human respiratory pathogens, causing annual epidemics that result in 3–5 million cases of severe illness and up to 650,000 deaths worldwide. Their ability to evade pre-existing immunity through continuous antigenic evolution and to generate pandemic strains through genetic reassortment makes them a persistent public health challenge.
Virion Structure and Classification
Influenza viruses belong to the Orthomyxoviridae family and are classified into four types: influenza A, B, C, and D. Influenza A and B cause most human disease and are responsible for seasonal epidemics, while influenza C causes mild illness and influenza D primarily infects cattle. The influenza A virion is enveloped with a segmented negative-sense RNA genome consisting of eight segments encoding at least 11 proteins. The envelope displays two major glycoproteins: hemagglutinin (HA), a trimeric spike protein that mediates viral attachment to sialic acid receptors and membrane fusion, and neuraminidase (NA), a tetrameric enzyme that cleaves sialic acid to release progeny virions from infected cells. The matrix protein M1 lines the inner envelope, and the ion channel M2 is embedded in the envelope. Influenza A is further classified into subtypes based on the antigenic properties of HA (H1–H18) and NA (N1–N11). The nucleoprotein (NP) encapsulates the RNA segments, and the viral RNA-dependent RNA polymerase is a heterotrimer of PA, PB1, and PB2 subunits.
Replication Cycle
Influenza virus entry begins with HA binding to sialic acid receptors on host epithelial cells, followed by receptor-mediated endocytosis. The acidic pH of the endosome triggers a conformational change in HA that exposes the fusion peptide, mediating fusion of the viral and endosomal membranes. The M2 ion channel acidifies the virion interior, releasing the ribonucleoprotein complexes into the cytoplasm. These complexes are imported into the nucleus, where the viral polymerase carries out transcription and replication. Primary transcription produces capped and polyadenylated mRNAs using cap snatching from host pre-mRNAs, mediated by the PB2 cap-binding and PA endonuclease activities. Replication proceeds through a complementary RNA intermediate and generates full-length progeny RNA segments. Newly synthesized viral RNPs are exported from the nucleus mediated by M1 and NS2/NEP proteins and assemble at the plasma membrane, where HA, NA, and M2 are embedded. Virions bud from the apical surface, with NA cleaving sialic acid to prevent virion aggregation and promote release.
Antigenic Drift and Shift
The continuous evolution of influenza viruses is driven by two mechanisms. Antigenic drift consists of accumulated point mutations in the HA and NA genes introduced by the error-prone viral RNA polymerase, which lacks proofreading activity (error rate approximately 10⁻³–10⁻⁴ per nucleotide per replication). Drift allows viruses to evade neutralizing antibodies elicited by prior infection or vaccination, necessitating annual reformulation of the seasonal influenza vaccine. Antigenic shift, occurring only in influenza A, involves reassortment of genome segments when two different influenza A strains infect the same cell. Reassortment can generate novel HA and NA combinations to which the human population has little pre-existing immunity, potentially causing pandemics. The 1918 H1N1 Spanish flu, 1957 H2N2 Asian flu, 1968 H3N2 Hong Kong flu, and 2009 H1N1 swine flu pandemics all resulted from antigenic shift events.
Influenza Pathogenesis
Influenza virus infects respiratory epithelial cells, causing ciliary dysfunction, epithelial cell death, and disruption of the airway barrier. Viral replication peaks 24–48 hours after infection, and symptoms including fever, cough, sore throat, myalgia, and headache result from both direct cytopathic effects and the host inflammatory response, with elevated levels of pro-inflammatory cytokines such as IL-6, TNF-α, and IFN-γ. Severe influenza can lead to primary viral pneumonia, secondary bacterial pneumonia (most commonly Streptococcus pneumoniae and Staphylococcus aureus), acute respiratory distress syndrome (ARDS), and multi-organ failure. Risk factors for severe disease include extremes of age, pregnancy, chronic respiratory or cardiovascular conditions, immunosuppression, and metabolic disorders such as obesity and diabetes.
Host Immune Response
The innate immune response to influenza is initiated by pattern recognition receptors including TLR3, TLR7, RIG-I, and NLRP3, which detect viral RNA and trigger production of type I and III interferons and pro-inflammatory cytokines. Interferon-induced antiviral effectors including Mx proteins, IFITM proteins, and OAS/RNase L restrict viral replication. The adaptive immune response involves neutralizing antibodies directed primarily against HA, which block receptor binding and fusion, and NA antibodies that limit viral spread. CD8+ cytotoxic T cells recognize conserved internal proteins such as NP, M1, and PB1, providing cross-reactive immunity across different influenza subtypes. The antigenic domain of HA, particularly the globular head, is under strong antibody selection pressure, while the stalk domain is more conserved and has been explored as a target for universal influenza vaccines.
Antiviral Drugs and Resistance
Two classes of antiviral drugs are approved for influenza treatment. Adamantanes (amantadine and rimantadine) block the M2 ion channel and are active only against influenza A, but widespread resistance has rendered them clinically ineffective. Neuraminidase inhibitors (oseltamivir, zanamivir, peramivir) are active against both influenza A and B and are the current standard of care. Oseltamivir resistance can emerge through mutations in the NA active site, most notably the H275Y substitution in N1 neuraminidase that was widespread in seasonal H1N1 before the 2009 pandemic. Baloxavir marboxil, a cap-dependent endonuclease inhibitor targeting the PA subunit, is a newer antiviral with a different resistance profile and single-dose oral administration.
