Seasonal Influenza

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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 between January and March/April and between July and 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 be further 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 humans and resulted in a pandemic in mid-2009. Influenza A (H1N1)pdm09 virus has now become one of the seasonal influenza strains worldwide.

Antigenic drift (minor changes) of seasonal influenza viruses can lead to the emergence of new strains. New strains appear from time to time and at irregular intervals, which necessitates annual recommendations by the World Health Organization on influenza vaccine composition.

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 pandemics have occurred roughly every 10 to 50 years in the past. Nonetheless, the timing of a future pandemic is difficult to predict. 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 a few days to less than two weeks. Symptoms may include fever, cough, sore throat, runny nose, muscle and joint pain, fatigue and headache; some may also have vomiting and diarrhoea.

However, seasonal influenza can cause serious illness in high-risk individuals, such as the elderly, children under 5 years of age, pregnant women and people with underlying medical conditions. Influenza-related complications include pneumonia, sepsis, encephalitis, myocarditis 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 to 4 days.

Management

• Most people recover from influenza on their own, but medical attention should be sought promptly if symptoms persist or deteriorate.
• People with mild symptoms should get adequate rest, drink plenty of water, and refrain from work or attending school.
• Antiviral agents may reduce the severity and duration of illness if started early, but a doctor’s prescription is required.
• Antibiotics work against bacteria, not viruses, and will neither cure influenza nor speed up recovery.

Prevention

1. Seasonal influenza vaccination

• The vaccine is safe and effective in preventing seasonal influenza and its complications.
• Annual seasonal influenza vaccination is recommended for all persons aged 6 months or above except those with known contraindications.
• Vaccination should generally be received every year in autumn. It takes about 2 weeks after vaccination for the body to 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

• Wearing a surgical mask is advised for the following situations:
  1. any person with fever or respiratory symptoms;
  2. any person who is 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 individuals (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 respiratory virus activity is expected, high-risk individuals are recommended to wear a surgical mask when visiting public places, while the public should also wear a surgical mask when taking public transport 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 https://www.chp.gov.hk/files/pdf/use_mask_properly.pdf;
• Perform hand hygiene frequently, especially before and after touching the mouth, nose or eyes; after touching public installations such as handrails or doorknobs; or when hands are contaminated by respiratory secretions 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. 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: https://www.chp.gov.hk/en/healthtopics/content/460/19728.html
• 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 immunity through a balanced diet, regular exercise, adequate rest, reducing stress, not smoking, and avoiding 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 to 30 minutes, and then rinse with water and keep dry. For metallic surfaces, 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 to 30 minutes and then rinse with water and keep dry. For metallic surfaces, disinfect with 70% alcohol.
• Maintain good indoor ventilation.

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