Influenza A is the most virulent flu virus, writes Dr Muiris Houston, Medical Correspondent
Influenza, commonly known as the flu, is a highly contagious viral infection of the respiratory tract affecting the nose, throat and lungs. Influenza A is one of three types of flu virus. A and B have the greatest impact on human health, with influenza A the type responsible for previous epidemics and pandemics. Type C is associated with sporadic and usually asymptomatic infection.
Of the three main types of flu, influenza A is the one which has always caused the greatest trouble. In different forms, it has been responsible for the world's greatest pandemics, including the infamous Spanish flu of 1918. The influenza virus can change its characteristics with relative ease. Minor changes in the surface of the virus - involving proteins called haemagglutinin (H) and neuraminidase (N) - occur between each influenza season.
These minor changes are referred to as antigenic drift and are the reason why it is necessary to reformulate the flu vaccine on an annual basis. When major alterations to the surface proteins occur, it is referred to as antigenic shift. A completely new virus then emerges, along with the likelihood of an influenza pandemic. Luckily, such major changes rarely occur.
The Fujian-like flu causing the present illness is classified as a H3N2 strain. It is slightly different to the H3N2 Panama-like virus which is one of the components of the World Health Organisation-approved vaccine currently available from your GP.
But because both contain the same principal proteins on their surfaces, the Panama-like influenza A vaccine should offer some protection against the latest sub-type now in circulation here.
Those for whom vaccination against the flu is advised will therefore still benefit from the current vaccine. It should, at the very least, reduce the severity of the infection.
Uptake amongst the over 65s, those with chronic heart and lung disease and people with diabetes is quite good. However, the Department of Health has expressed concern about a low uptake amongst healthcare workers. As a group, they are about 10 times more likely to contract flu compared with the general population. In addition, they are a potential source of infection for older people and others for whom influenza remains a potentially fatal condition.
The current strain of influenza A appears to be affecting children in greater numbers than adults. According to the National Disease Surveillance Centre, this is because there has been very little influenza in circulation for the last few seasons, therefore the opportunity for development of immunity has been limited.
In Japan in 1998 and 1999, during a large outbreak of influenza A, the possibility of the virus causing a massive infection in children's brains was recognised. The outbreak mainly affected children under five. Of the many children affected only a tiny number developed altered consciousness and convulsions within 24 hours of the illness. It appears that this week's tragic death of an 18-month-old boy in the Republic is an example of this rare occurrence.
The symptoms of flu are more severe than those of the common cold. Typically, flu symptoms come on within a couple of hours and usually leave the victim feeling pole-axed.
The main symptoms of influenza are: high fever; chills; headache and muscle ache; dry throat; dizziness; tiredness and loss of appetite. Cough, sore throat, a runny nose and nausea may also occur.
In young children who cannot readily communicate symptoms, parents should look out for drowsiness, a child who has gone off their food or difficulty breathing. If in any doubt about the possibility of an infant having influenza, contact your GP.
Flu symptoms usually begin to settle after five days or so. However, it can take a good deal longer for tiredness and a feeling of being "below par" to disappear.
The severe complications of influenza are rare. Encephalitis, an inflammation of the brain tissue, occurs in less than 0.1 per cent of cases. Unfortunately, however, it can be fatal. Rarely, the virus attacks the muscle of the heart, leading to a condition called myocarditis. But for the overwhelming majority of people who contract the flu, it is not a life-threatening illness.