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Epidemiology

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

Authoring team

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

Reference:

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

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