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Seasonal Influenza

Seasonal Influenza

22 December 2023

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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/April 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 attending class at 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.


1. 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.

2. Maintain good personal hygiene

  • The wearing of mask is advised for the following situations - 
    1. any persons with fever or respiratory symptoms;
    2. any persons who are visiting or working in high risk places such as residential care homes for the elderly, residential care homes for persons with disabilities, and clinical areas of healthcare facilities; and
    3. high risk persons (e.g. persons with underlying medical conditions or persons who are immunocompromised) who are going to crowded places such as public transport.
    4. When an increasing trend in activity of respiratory viruses is expected, high risk persons are recommended to wear a surgical mask when visiting public places, while the public should also wear a surgical mask when taking public transportations or staying at crowded places.
  • It is important to wear a mask properly, including hand hygiene before wearing and after removing a mask. For details on proper mask wearing, please visit CHP website at;
  • Perform hand hygiene frequently, especially before and after touching the mouth, nose or eyes; after touching public installations such as handrails or door knobs; or when hands are contaminated by respiratory secretion after coughing or sneezing.
  • Wash hands with liquid soap and water, and rub for at least 20 seconds. Then rinse with water and dry with either a clean cotton towel or a paper towel. If hand washing facilities are not available, or when hands are not visibly soiled, hand hygiene with 70 to 80% alcohol-based handrub is an effective alternative. To perform hand hygiene properly, please browse the following webpage:
  • Cover your mouth and nose with tissue paper when coughing or sneezing. Dispose of soiled tissues into a lidded rubbish bin, then wash hands thoroughly.
  • When having respiratory symptoms, wear a surgical mask, refrain from work or attending class at school, avoid going to crowded places and seek medical advice promptly.
  • Build up good body immunity by having a balanced diet, regular exercise, adequate rest, reducing stress, do not smoke and avoid alcohol consumption. 

3. Maintain good environmental hygiene

  • Regularly clean and disinfect frequently touched surfaces such as furniture, toys and commonly shared items with 1:99 diluted household bleach (mixing 1 part of 5.25% bleach with 99 parts of water), leave for 15-30 minutes, and then rinse with water and keep dry. For metallic surface, disinfect with 70% alcohol.
  • Use absorbent disposable towels to wipe away obvious contaminants such as respiratory secretions, and then disinfect the surface and neighbouring areas with 1:49 diluted household bleach (mixing 1 part of 5.25% bleach with 49 parts of water), leave for 15-30 minutes and then rinse with water and keep dry. For metallic surface, disinfect with 70% alcohol.
  • Maintain good indoor ventilation. Avoid going to crowded or poorly ventilated public places; high-risk individuals may consider putting on surgical masks while in such places.

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