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.
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.
You should put on a surgical mask if you develop respiratory symptoms such as fever, cough, sore throat, runny nose, muscle pain, fatigue and headache; and should have adequate rest, drink plenty of water, refrain from work or school. You should seek medical advice if symptoms persist or deteriorate so that a diagnosis could be made early. Antibiotics, which target bacterial infection but not viral infection, will not cure influenza or make recovery faster. In addition, antiviral agents may reduce severity and duration of illness but must be used under doctor's prescription.
4. How is seasonal influenza transmitted?
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.
To prevent getting seasonal influenza, members of public are advised to:
Remember to wash your hands under the following conditions:
In general, members of the public should wash hands with soap and water when hands are visibly dirty or visible soiled with blood, body fluid, after using the toilet or changing the diapers. When hands are not visibly soiled, clean them with 70 - 80% alcohol-based handrub as an effective alternative.
Members of public should wear a surgical mask when respiratory symptoms develop, e.g. fever, runny nose, coughing or sneezing; or care for patients with respiratory symptoms and visit clinics or hospital. 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.
Given influenza vaccines are safe and effective and severe cases can occur even in healthy persons, all members of the public aged 6 months or above, except those with known contraindications, should receive seasonal influenza vaccine for personal protection.
Both trivalent (IIV3) and quadrivalent (IIV4) inactivated influenza vaccines are recommended for use in Hong Kong. Trivalent influenza vaccine may potentially prevent the majority of influenza burden in Hong Kong, while quadrivalent influenza vaccine may potentially offer additional protection against influenza B. Members of the public should consult their family doctors for details.
Yes. The circulating seasonal influenza strains may change from time to time. In accordance with the circulating strains, the seasonal influenza vaccine composition is updated every year to enhance protection. The immunity built up in a vaccinated person in the prior season will decrease over time and may become too low to provide protection in next season.
Inactivated seasonal influenza vaccine is very safe and usually well tolerated apart from occasional soreness, redness or swelling at the injection site. Some recipients may experience fever, muscle pain, and tiredness beginning 6 – 12 hours after vaccination and lasting up to 2 days. If fever or discomforts persist, please consult a doctor. Immediate severe allergic reactions like hives, swelling of the lips or tongue, and difficulties in breathing are rare and require emergency consultation.
Influenza vaccination may be rarely followed by serious adverse events such as Guillain-Barré syndrome (about 1 to 2 case per million vaccinees) and severe allergic reaction (anaphylaxis) (9 in 10 million doses distributed). However, influenza vaccination may not necessarily have causal relations with these adverse events. Studies have shown that the risk of Guillain-Barré syndrome after influenza infection (17.20 per million) is much higher than after influenza vaccination (1.03 per million).
(The Lancet Infectious Diseases. 2013 Sep; 13(9): 769-76)