What is influenza?

Influenza is an acute respiratory illness caused by influenza virus. The peak of influenza in Hong Kong is usually from January to March and from July to August. The outbreak of influenza in these periods is called seasonal influenza. The incubation period is 1-4 days. Symptoms are fever, cough, sore throat, runny nose, muscle pain, fatigue, headache, vomiting, and diarrhea. There are three types of influenza: A, B, and C. Influenza A can be further subtyped based on its surface antigens: haemagglutinin (H) and neuraminidase (N). H5N1, H3N1, and H1N1 are the most common influenza A. When the surface antigens have significant changes, a new subtype will emerge. Human usually have no immunity against the newly emerged subtypes, which allows the new subtypes to spread.


Transmission of influenza

Influenza virus is mainly spread through droplets generated by coughing, sneezing, and talking. Direct contact with the patients’ secretions is also a way to transmit influenza. The virus becomes infectious 1 day before and up to 5 – 7 days after symptoms are developed. Infants or patients whose immunity systems are severely deficient have a longer infectious period.


Prevention of influenza

People who are infected with influenza should normally be recovered in 2-7 days. Few will develop into severe conditions. Elderly or the weak may develop complications like bronchitis and pneumonia, or even death.

The inactivated trivalent (3-in-1) influenza vaccine gives the most protection to people. Its efficacy reaches 70-90%. The best time of vaccination should be in autumn. After two weeks, the body will develop enough antibodies to fight against the virus.

The Scientific Committee on Vaccine Preventable Diseases recommends the vaccine in 2013/14 should contain the following:

  • an A/California/7/2009 (H1N1)-like virus
  • an A(H3N2) virus antigenically like the cell-propagated prototype virus A/Victoria/361/2011
  • a B/ Massachusetts /2/2012-like virus

If quadrivalent (4-in-1) influenza vaccine becomes available, the above three viruses and a B/Brisbane/60/2008-like virus should be included.

Other preventions:

  • Cover your mouth and nose with tissue paper when coughing and sneezing
  • Maintain good personal and environmental hygiene
  • Put on a surgical mask if respiratory symptoms appear
  • Clean your hands with alcohol-based hand rub
  • Maintain good indoor ventilation
  • Avoid staying in crowded places
  • Proper diet
  • Regular exercise
  • Sufficient rest
  • Don’t smoke
  • Avoid stress overload


Who should be vaccinated?

  • Long stay residents of residential care homes or institutions for the disabled
  • Aged 6 months to under 6 and 50 or above
  • People with chronic medical problems, e.g. obesity (BMI 30 or above), long-term cardiovascular problem, lung problem, metabolic problem or kidney problem, and low immunity
  • Health care professionals
  • Poultry workers
  • Pig farmers and pig-slaughtering industry personnel


What are the common side effects?

The common side effects are the pain and redness caused by the injection, but it will wear off soon. Few people will develop fever, fatigue, muscle pain, and joint pain in 6 to 12 hours, but if so, these will also go off in two days. If the side effects lasts long or evolves into severe reaction, immediate consultation to a doctor is required.


Who are not recommended to get vaccinated?

  • People who are allergic to inactivated influenza vaccine
  • People with middle to severe illness



  1. Centre for Health Protection, Department of Health
  2. Centre for Health Protection, Department of Health – Influenza Vaccination
  3. Centre for Health Protection, Department of Health – Frequently Asked
  4. Department of Health, Australian Government
  5. Department of Health, Australian Government – Influenza Immunization Program
  6. Centers for Disease Control and Prevention – Seasonal Influenza Vaccination Resources for Health Professionals
  7. U.S. food and Drug Administration