Influenza: treatment and prevention

Influenza affects 10–20 per cent of the global population. The influenza virus belongs to the orthomyxovirus family. It is made up of a lipid membrane surrounding a protein shell and a core containing eight RNA segments. There are three types of influenza virus (A, B and C), based on the protein composition of the viral envelope. Annual flu epidemics are caused by types A or B, with the type B being identified as dominant last year (75 per cent of flu infections in the UK were caused by influenza B). Influenza B usually causes minor symptoms but infection can be severe for older people. Influenza viruses can be further categorised according to the classes of surface proteins they express. Subtypes of influenza A are differentiated by the variability of their surface glycoproteins (haemagglutinin [H] and neuraminidase [N]). There are 16 types of haemagglutinin and nine types of neur-aminidase. These antigenic variants are named according to their H and N antigen types. The main subtypes known to cause influenza are H1N1, H2N2 and H3N2. All influenza viruses undergo mutation. Point mutations occur in the amino acid sequences of the haemagglutinin and neuraminidase glycoproteins and this results in slight modifications (antigenic drift) in the virus, allowing for the development of new strains within a subtype. Viral strains are named according to the virus type, geographical origin, strain number, year of isolation and subtype.