What is influenza?
Influenza is an infectious disease caused by various types of influenza virus. In Hong Kong, the two subtypes of influenza A virus, H1N1 and H3N2, are most commonly seen. Influenza occurs in Hong Kong throughout the year with seasonal peaks most commonly in January to March and sometimes July to August. Influenza is transmitted via the respiratory tract, predominantly by airborne or droplet spread or through direct contact with patients' secretions. The disease is characterised by fever, sore throat, cough, headache, muscle aches, runny nose and general tiredness. It is usually self-limiting with recovery in two to seven days. However, it can be a serious illness to the weak and frail, such as elderly persons, and may be complicated by bronchitis, pneumonia or even death in the most serious cases.
Why is influenza vaccination important?
Influenza occurs in Hong Kong throughout the year. Influenza causes significant disease burden in some population groups where it is associated with increased risk of complications. Influenza vaccination is important because it is one of the effective means in preventing influenza and its complications.
Who should receive influenza vaccination?
In 2009, the Scientific Committee on Vaccine Preventable Diseases (SCVPD) recommends the following target groups for influenza vaccination in Hong Kong:
* People with chronic illness mainly refer to those who have chronic circulatory, lung, metabolic or renal diseases, those whose immune response are reduced or defective, and those with chronic neurological disorders that can compromise respiratory function or the handling of respiratory secretions or that can increase the risk for aspiration or those who lack the ability to care for themselves.
When to get vaccinated?
In Hong Kong, influenza peaks most commonly around January to March and July to August. The SCVPD recommends influenza vaccination to be given at least two weeks prior to the anticipated seasonal peak of influenza.
What kinds of influenza vaccines are there?
There are two types of vaccines that protect against influenza. The 'flu shot' is an inactivated vaccine (containing killed virus) that is given with a needle. A different kind of vaccine, called the nasal-spray flu vaccine (sometimes referred to as LAIV for Live Attenuated Influenza Vaccine) is available overseas, but not licensed in Hong Kong.
What is the recommended influenza vaccine composition?
The SCVPD advised vaccines to be used are vaccines recommended by the World Health Organization in the 2009-2010 season (northern hemisphere winter), which contain:
Who should not receive influenza vaccination?
People who are hypersensitivity to eggs, neomycin, a previous dose of influenza vaccine or other vaccine components are not suitable to have the influenza vaccination. Those with bleeding disorders or on warfarin may receive the vaccine by deep subcutaneous injection. Individuals who suffer from an acute febrile illness on the day of vaccination may receive the vaccine when they recover. The vaccine can be given on the same day as other types of vaccines.
As it is unknown whether influenza vaccination is causally associated with increased risk of recurrent Guillain-Barre Syndrome (GBS), persons with previous history of GBS are advised to consult their doctors before receiving the vaccine.
Does the vaccine cause influenza?
As the viruses in the vaccine are inactivated, the vaccine cannot cause influenza. However, some people do develop side effects after vaccination.
Can pregnant women receive influenza vaccination?
Influenza vaccination with inactivated influenza vaccine in pregnancy is considered safe by the World Health Organization and there is no evidence indicating that inactivated influenza vaccine is teratogenic even if given during the first trimester.
What are the possible side effects of the vaccine?
Influenza vaccine is usually well tolerated apart from occasional soreness at the injection site. The recipient may experience fever, muscle and joint pains, and tiredness beginning 6 to 12 hours after vaccination and lasting up to two days. Immediate severe allergic reactions like hives, swelling of the lips or tongue, and difficulties in breathing are rare and require emergency consultation. If fever persists despite taking paracetamol, or there are other symptoms or reactions, please consult a doctor.
Does the vaccine work right away?
No. It takes about two weeks after vaccination for antibodies to develop in the body and provide protection against influenza virus infection.
How do influenza vaccines work?
The vaccine induces development of antibodies against influenza virus infection in the body.
How long will the protection from the vaccine last?
The vaccine currently in use helps to reduce the chance of influenza and its complications, but it does not offer 100% protection. The immunity built up in the vaccinated person will decline over time and may be too low to provide protection after one year. Vaccinated persons may still develop influenza, especially when the circulating influenza viruses differ significantly from the vaccine strains. For prevention against influenza, vaccinated individuals still need to maintain good personal and environmental hygiene practices, balanced diet, regular exercise, adequate rest, and avoid smoking.
Should I need to get vaccinated for influenza every year?
Yes. New subtype variants appear from time to time and at irregular intervals. This is responsible for seasonal outbreaks and implies:
Department of Health, HKSAR