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What is influenza?

  • Influenza is caused by a virus that attacks mainly the upper respiratory tract – the nose, throat and bronchi and rarely also the lungs

  • Hospitalization and deaths mainly occur in high-risk groups (elderly, chronically ill)

  • More prevalent in January-March and July-August each year in HK

Type of influenza

Three types of influenza viruses are recognized: A, B and C

The currently circulating influenza viruses that cause human disease are type A and B

Type A

  • Influenza A has 2 subtypes which are important for humans: A(H3N2) and A(H1N1), of which the former associated with most death.

  • It is infectious to mammals e.g. pig, birds and human

  • Emergence of new subtypes occurs from time to time at irregular intervals

Type B

  • Also common in HK and is infectious to human only

Type C

  • Less commonly seen in HK

  • Can be epidemic 

Causes - How is influenza transmitted?

  • Via air droplets

  • Small particles from cough and sneeze

  • The influenza virus enters the body through the nose or throat

  • Disease spreads very quickly among the population especially in crowded circumstances

Clinical symptoms

  • Incubation period around 1 to 3 days

  • Three main symptoms, namely fever, sore throat and cough

  • Headache, muscle pain, runny nose and other symptoms like tiredness, diarrhea and vomiting may also appear

  • Usually self-limiting with recovery in 2 to 7 days


Difference between influenza and common cold









Managing influenza

  • Infected persons should take adequate rest and drink plenty of water

  • Antiviral agents can reduce the severity and duration of illness e.g. amantadine is effective for strains of influenza A    

  • Antibiotics are unnecessary unless influenza is complicated by bacterial infection

Prevention of Influenza

  • Maintain good personal and environmental hygiene.

  • Build up body resistance by balanced diet and regular exercise

  • Avoid crowded environment during flu season

  • Vaccination - Recommended to receive each year to maintain the protection

Who should not receive the vaccine?

  • Individuals with egg protein or vaccine components allergies

  • With fever or infection on day of vaccination

  • Infants under 6 months old

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