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Epidemics of influenza are associated with antigenic drift which results in failure of recognition by the immune system.

  • the severity of these annual epidemics may vary according to the types, subtypes and strains of circulating viruses, and the level of protective antibodies in the general population (1)
  • peak incidence in influenza usually occurs
    • May to September - in temperate regions of the southern hemisphere
    • December and March and lasts for 6–8 weeks - in temperate regions of the northern hemisphere
    • variable times of the year in tropical and subtropical countries and can cause isolated cases because influenza viruses in these regions circulate at low levels throughout the year (1)
  • epidemics take 4 weeks to peak and last for 8–10 weeks.

Pandemics of influenza occur as a result of antigenic shift throwing up an entirely new combination of antigens.

  • influenza infection rates are generally higher in all age groups during pandemics than during annual epidemics
  • school-age children play an important role in the spread of the disease in the community
  • three pandemics have occurred during the 20th century; In all instances, the spread of the infection throughout the world was seen within a year of its initial detection
    • 1918–1919 the “Spanish flu” A(H1N1) - caused death to more than 40 million people
    • 1957–1958 the “Asian flu” A(H2N2) - responsible for more than 1 million deaths globally
    • 1968–1969 “Hong Kong flu” A(H3N2) - mortality rate was estimated to be around 1 million
  • the pandemic (H1N1) in 2009 had spread to all six WHO regions within 6 weeks of first being described which resulted in the declaration of a pandemic

GP consultations for influenza vary according to the prevalence of the disease:

  • 30 consultations/100000 population is a winter baseline
  • 100-150 consultations/100000 are seen in an epidemic
  • 600 consultations/100000 represents a severe outbreak


  1. World Health Organization (WHO). WHO Global Influenza Surveillance Network. Manual for the laboratory diagnosis and virological surveillance of influenza

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