Influenza ("flu") is a common viral disease affecting the respiratory system, causing fever, headache, muscle aches & pains, sore throat & cough. It is easily spread from person to person by airborne transmission. Secondary pneumonia may occur, especially in older people or people with chronic medical conditions which may make them more vulnerable. Outbreaks occur in most years during spring & winter (i.e. April until September in the southern hemisphere & October until March in the northern hemisphere). In the tropics, influenza can occur throughout the year.
The influenza virus undergoes minor changes in its structure from year to year, so that every year the formulation of the flu vaccine needs to be reviewed to ensure that it provides protection against the current flu strains. Every ten to thirty years, a new subtype emerges which causes a flu epidemic (or pandemic) which may affect a quarter or more of the population.
The flu vaccine is a killed vaccine which is recommended every year in autumn. While it used to be recommende just for people aged over 65 years, & anybody with chronic medical problems, it is now accepted that everyone can benefit from annual influenza vaccine, including children. The vaccine confers about 70% protection for about one year.
Travellers may be exposed to an increased risk of exposure to the flu virus. This is particularly true for those travelling in large tourist groups or on cruise ships. The virus is transmitted all year-around in the tropics, so travellers can be avaccinated anytime.
The vaccine should not be given to people with an anaphylactic hypersensitivity to eggs, or to anybody with a high fever. It should be used with caution in anybody with a history of Guillain-Barre syndrome.
As the flu vaccine is not a live vaccine, it does not cause a vaccinated person to develop influenza. However, occasionally somebody who has been vaccinated may feel unwell & experience fever and/or aches & pains for a day or two after the vaccine. Allergic reactions are very rare.
Anti-viral drugs can also be useful for prevention or treatment of influenza, & both oseltmivir & zanamivir are currently available. For prevention, they can be used after a contact with a proven case of influenza, & for treatment, they are effective when taken early in the illness. These can only be obtained on prescription in Australia at present.