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Seasonal Influenza

Seasonal Influenza

14 June 2018

Causative agents

Seasonal influenza is an acute illness of the respiratory tract caused by influenza viruses. It is usually more common in periods from January to March and from July to August in Hong Kong. Three types of seasonal influenza viruses are recognised to cause human infection, namely A, B and C. Influenza A viruses can further be subtyped on the basis of two surface antigens: haemagglutinin (H) and neuraminidase (N). Currently, there are two subtypes of seasonal influenza A viruses circulating in humans, namely influenza A (H1N1)pdm09 and influenza A (H3N2). In the spring of 2009, influenza A (H1N1)pdm09 virus emerged to cause illness in human and resulted in a pandemic in mid 2009. Influenza A (H1N1)pdm09 virus has now become one of the seasonal influenza strains worldwide.

Antigenic drifts (minor changes) of seasonal influenza viruses will lead to emergence of new strains. New strains appear from time to time and at irregular intervals. This explains why the World Health Organization will make recommendations on the formulation of influenza vaccine every year.

Seasonal influenza is different from influenza pandemic. The occurrence of influenza pandemic results from the emergence of a pandemic strain, which appears when antigenic shift (major change) occurs in a novel influenza virus resulting in efficient human-to-human transmission. People will have very little or no immunity to the virus, which causes the virus to spread more rapidly and extensively than a seasonal influenza virus.

Influenza pandemic occurred roughly every 10 – 50 years in the past. Nonetheless, the timing of a future pandemic cannot be predicted. It is usually associated with a large number of cases, higher severity of illness, a higher death toll and consequently greater social and economic disruption.

Clinical features

For healthy individuals, seasonal influenza is usually self-limiting with recovery in 2 – 7 days. Symptoms may include fever, cough, sore throat, runny nose, muscle pain, fatigue and headache; some may also have vomiting and diarrhoea. 

Cough is often severe and prolonged but fever and other symptoms generally resolve in 5 – 7 days. However, influenza can be a serious illness to the weak and frail or elderly people, and may be complicated by bronchitis, chest infection or even death.

Mode of transmission

Influenza viruses mainly spread through droplets when infected people cough, sneeze or talk. The infection may also spread by direct contact with the secretions of infected persons.

Incubation period

Usually around 1 – 4 days.

Infectious period

Infected persons may pass the viruses to other people 1 day before and up to 5 – 7 days after they develop symptoms. The period may be even longer in young children or severely immunocompromised persons.


  • Have adequate rest and drink plenty of water.
  • Refrain from work or school when having symptoms of influenza. Seek medical advice if symptoms persist or deteriorate.
  • Antibiotics which target bacterial infection but not viral infection will not cure influenza or make recovery faster.
  • Antiviral agents may reduce severity and duration of illness but must be used under doctor's prescription.


Seasonal influenza vaccination

  • The vaccine is safe and effective in preventing seasonal influenza and its complications.
  • Influenza can cause serious illnesses in high-risk individuals and even healthy persons. Given that seasonal influenza vaccines are safe and effective, all persons aged 6 months or above except those with known contraindications are recommended to receive influenza vaccine for personal protection.
  • Usually, it is suggested that vaccination should be received in autumn every year. About 2 weeks after vaccination, the body will develop a sufficient level of antibodies to protect against influenza virus infection.

*For details on seasonal influenza vaccination, please refer to Vaccination Schemes and Vaccination for Seasonal Influenza.

Personal and environmental hygiene

  • Wash hands frequently with liquid soap and water, especially before touching the mouth, nose or eyes, or after touching public installations such as handrails or door knobs.
  • When hands are not visibly soiled, clean them with 70 - 80% alcohol-based handrub as an effective alternative.
  • Cover nose and mouth when sneezing or coughing, and wash hands thoroughly afterwards.
  • Dispose of soiled tissue paper properly in a lidded rubbish bin.
  • Put on a surgical mask when having respiratory symptoms.
  • Maintain good indoor ventilation.
  • When influenza is prevalent, avoid going to crowded or poorly ventilated public places; high-risk individuals may consider putting on surgical masks while in such places.
  • Maintain a balanced diet, exercise regularly, take adequate rest, do not smoke and avoid overstress. 

For details, please refer to the thematic webpage of Seasonal Influenza.

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