Influenza (seasonal)

Influenza (flu) is a highly infectious respiratory virus. According to the World Health Organisation (WHO), it is responsible for more than a one billion cases annually with 290,000 to 650,000 deaths. There are four types of influenza viruses (A, B, C, D), but only Influenza A and B are important for public health purposes.

Influenza viruses spread from person to person by droplet infection or small particle aerosol. Droplets generally travel only a short distance (< 2 metres) while aerosol may go further.

The virus replicates in and infects adjacent cells of the upper respiratory tract. Local mucosal immunity develops and there is a rise in serum antibodies.

Individuals are most contagious in the first 3 days after the onset of symptoms but probably excrete the virus for about 5 days. Those who are immune-suppressed or who have chronic disease excrete the virus longer.

The virus can remain viable for hours in dried secretions before transfer to the respiratory tract. The incubation period is 1 to 2 days.

Influenza is NOT a benign disease and is often confused with the cold (rhinovirus, adenovirus etc). Colds are common. This is why they are called the “common cold”. Influenza is not so common. Individuals might experience a true bout of influenza just three or four times in a lifetime. Influenza is more likely to put you into bed for a few days, cause a high temperature and chills and elicit unpleasant symptoms like a hacking cough. You wouldn’t be a welcome dinner guest if you had flu-like symptoms.

Laboratory confirmed influenza is notified in Australia. Almost 300,000 cases have been reported in the year to date (June 2024). This is up on the 5-year rolling average but takes in the pandemic years where the rates were low.

Primary school-age children are seen as the group most likely to transmit the disease in the community. Of concern is the lower immunization rates (since 2020) in those under 5 years.

More than 1,000 individuals die from the complications of influenza annually. Most of these are over 65 years or under 5 years. Many have contributing factors which make the illness worse.

There is a worldwide distribution for influenza, but exposure risk depends on many factors such as time of travel, type of travel and activities relating to the stay. Infection rates are highest in children (the likely non immunes). Children under 18 under twice as likely to contract influenza than adults over 65 years but more severe illnesses are found in those over 65 years (hence national immunization program).

Attack rates are high in a susceptible population. This can be as high as 25% in a community setting and more than 40% in closed settings such as a school, retirement village or nursing home. A “community setting” could be a cruise ship!

The peak activity is in the winter months in temperate climates and during the dry season of the tropics and subtropics.

The administration of an influenza vaccine to persons at risk is the most important measure in prevention of illness or the severe complications.

Travellers are always at risk (airports terminals, commuter transport, crowds). Many are elderly and some have significant co-morbidities (diabetes, chronic lung disease, cardiovascular disease).