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. 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. 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 is 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 is safe and effective in preventing seasonal influenza and its complications. Influenza can cause serious illnesses in high-risk individuals and even healthy persons. Given that seasonal influenza vaccines are safe and effective, all persons aged 6 months or above except those with known contraindications are recommended to receive influenza vaccine for personal protection. Usually, it is suggested that vaccination should be received in autumn every year. About 2 weeks after vaccination, the body will develop a sufficient level of antibodies to protect against influenza virus infection.
* The above information on seasonal influenza vaccine is provided by Centre for Health Protection (CHP) of the Department of Health on their website.