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Diseases / Influenza


What is it?: 

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.1

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. 1

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.1

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.1

Incubation period: 

Usually around 1 – 4 days.1


Seasonal influenza vaccination1

  • 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.


Personal and environmental hygiene1

  • 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.
  • Have adequate rest and drink plenty of water. 1
  • Refrain from work or school when having symptoms of influenza. Seek medical advice if symptoms persist or deteriorate. 1
  • Antibiotics which target bacterial infection but not viral infection will not cure influenza or make recovery faster. 1
  • Antiviral agents may reduce severity and duration of illness but must be used under doctor's prescription.1
High risk groups: 

People who are in the priority groups are generally at increased risk of severe influenza or transmitting influenza to those at high risk. Therefore, they should have higher priority for seasonal influenza vaccination.

In the 2020-21 season, the Scientific Committee on Vaccine Preventable Diseases (SCVPD) recommends the following priority groups for seasonal influenza vaccination in Hong Kong2:

  • Pregnant women
  • Residents of Residential Care Homes (such as Residential Care Homes for the Elderly [RCHE] or Residential Care Homes for Persons with Disabilities [RCHD])
  • Persons aged 50 years or above
  • Persons with chronic medical problems*
  • Healthcare workers (including care workers in RCHEs and RCHDs)
  • Children aged 6 months to 11 years
  • Poultry workers
  • Pig farmers and pig-slaughtering industry personnel
* People with chronic medical problems mainly refer to those who have chronic cardiovascular (except hypertension without complication), lung, metabolic or kidney diseases, obesity# (Body Mass Index [BMI] 30 or above), who are immunocompromised^, children and adolescents (aged 6 months to 18 years) on long-term aspirin therapy, and those with chronic neurological condition that can compromise respiratory function or the handling of respiratory secretions or that can increase the risk for aspiration or those who lack the ability to care for themselves.

# Obesity is considered as an independent risk factor for influenza complication and thus people with BMI 30 or above are included in the priority groups for seasonal influenza vaccination.

^ People who are immunocompromised refer to those with a weakened immune system due to disease (such as HIV/AIDS) or treatment (such as cancer treatment).



1.CHP Flu. Accessed on 18Feb2021

2.CHP Flu FAQ. Accessed on 18Feb2021