Seasonal Influenza

Disease Information

Seasonal Influenza

Seasonal Influenza

Causative agent

Influenza (flu) 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 locally.  Three types of influenza viruses are recognized, namely A, B and C.  Influenza A viruses can further be subtyped and influenza A (H1N1) 2009 virus (i.e. human swine influenza virus) is one of them.  In the spring of 2009, influenza A (H1N1) 2009 virus emerged to cause illness in human and resulted in a pandemic in mid 2009.  In August 2010, the World Health Organization (WHO) announced that the world had moved into the post-pandemic period and the virus was expected to continue to circulate in the community as a seasonal influenza strain for some years to come.  Influenza A (H1N1) 2009 virus has now become one of the seasonal influenza strains in Hong Kong.

Clinical features

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 protracted 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 persons, 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 they develop symptoms and up to 5 – 7 days after. The period may be even longer in young children or severely immunocompromised persons.

Management

  • Have adequate rest and drink plenty of water
  • Refrain from work or school when having symptoms of influenza. Seek medical advice if symptoms persist
  • 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

Prevention

Receive seasonal influenza vaccination

  • The vaccine is safe and effective in preventing seasonal influenza and its complications
  • As serious influenza can occur even in healthy individuals, members of the public can consult their family doctors to receive seasonal influenza vaccination for personal protection
  • Emergence of new strains occurs from time to time at irregular intervals, which may cause outbreaks. Therefore, the WHO recommends appropriate formulation of influenza vaccine for every influenza season

Please refer to UHS website for further information and vaccination schedule:

Keep hands clean

  • Wash hands with liquid soap and water properly
  • When hands are not visibly soiled, clean hands with 70 - 80% alcohol-based handrub

Maintain cough manners

  • Cover nose and mouth when sneezing or coughing
  • Dispose of soiled tissue paper properly in a lidded rubbish bin
  • Wash hands thoroughly after sneezing or coughing
  • Put on a surgical mask when having respiratory symptoms

Observe environmental hygiene

  • Maintain good indoor ventilation
  • When influenza is prevalent, avoid going to crowded or poorly ventilated public places

In addition, members of the public should also maintain balanced diet, exercise regularly, take adequate rest, do not smoke and avoid overstress. 

 

Related information:

Influenza Page
 

Source: Centre for Health Protection HKSAR (22 Nov 2011)