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. Influenza A viruses can further be 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 human and resulted in a pandemic in mid 2009. Influenza A (H1N1)pdm09 virus has now become one of the seasonal influenza strains worldwide.
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.
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.
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.
Usually around 1 – 4 days.
Infected persons may pass the viruses to other people 1 day before and up to 5 – 7 days after they develop symptoms. The period may be even longer in young children or severely immunocompromised persons.
Seasonal influenza vaccination
*For details on seasonal influenza vaccination, please refer to Vaccination Schemes.
Personal and environmental hygiene
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