Influenza A viruses are members of the Orthomyxoviridae family and consist of enveloped, segmented, negative‑sense RNA viruses that infect birds and mammals. They cause seasonal flu in humans and are the only influenza viruses capable of causing pandemics.
Virology & Pathogenesis
Influenza A viruses have eight RNA segments encoding structural proteins such as hemagglutinin (HA), neuraminidase (NA), nucleoprotein, and matrix proteins, along with several non‑structural proteins. They are classified into subtypes based on their HA and NA glycoproteins; there are 18 HA subtypes and 11 NA subtypes, and specific combinations circulate among avian, swine and human populations. According to the CDC, influenza A viruses are the only influenza viruses known to cause flu pandemics. Infection begins when HA binds to sialic acid residues on respiratory epithelial cells, followed by endocytosis, uncoating and replication in the host cell nucleus. Because the genome is segmented, co‑infection with different strains permits reassortment, producing novel viruses through antigenic shift. Antigenic drift—accumulation of point mutations in HA and NA—drives continual evolution and necessitates yearly vaccine reformulation. Wild aquatic birds serve as the major reservoir, but influenza A viruses also infect pigs, horses, dogs and marine mammals, enabling cross‑species transmission and occasional zoonotic emergence. Current human seasonal influenza A viruses include A(H1N1) and A(H3N2) subtypes.
Major subtypes and impacts
Notable influenza A subtypes illustrate the virus’s diversity. The 1918 A(H1N1) strain caused the catastrophic “Spanish flu” pandemic; later pandemics included A(H2N2) in 1957 and A(H3N2) in 1968. In 2009 a novel A(H1N1)pdm09 virus emerged from swine reassortment and quickly spread worldwide. Highly pathogenic avian influenza viruses such as H5N1 and H7N9 infect birds and occasionally humans, causing severe disease and prompting culling of poultry flocks. Low‑pathogenic avian strains circulate widely in waterfowl, contributing genetic segments to emergent viruses. Surveillance of animal reservoirs and human cases is essential to detect antigenic shifts early, inform vaccine development and guide public health responses. Vaccination, antiviral drugs like neuraminidase inhibitors, and non‑pharmaceutical measures reduce disease burden during seasonal epidemics and pandemics.
Influenza A viruses remain a major global public health concern because of their capacity for rapid evolution and cross‑species transmission. Ongoing surveillance and vaccine updates are critical to mitigate the impact of seasonal epidemics and prevent or contain future pandemics.
Related Terms: Orthomyxoviridae, Hemagglutinin, Neuraminidase, Antigenic drift, Antigenic shift