In most cases influenza (or ‘flu’) is a benign illness, but for elderly people, pregnant women and people with chronic diseases, the complications of flu can be dangerous. Vaccination remains the best way of preventing the complications of flu and hospitalisation.

What is influenza or ‘flu’?

Influenza or ‘flu’ is an infectious illness caused by the influenza virus.

It appears in the form of winter epidemics, generally between December and April in the Northern hemisphere and between May and September in the Southern hemisphere.

The influenza A and B viruses are the cause of these seasonal epidemics, but only the influenza A virus is responsible for pandemics.

What is a flu epidemic?

An epidemic means the occurrence of a disease in excess of normal expectancy in a given population.

To speak of an influenza epidemic certain conditions must be met:

  • the number of doctor visits for the influenza syndrome must be higher than the epidemic threshold (in Belgium this threshold is about 140 doctor visits for 100,000 inhabitants per week)
  • the epidemic threshold must be exceeded for at least 2 consecutive weeks
  • the virological analyses must show that at least 20% of respiratory tract samples analysed are positive for influenza virus.

A seasonal epidemic of influenza lasts on average between 6 and 12 weeks.

What is a flu pandemic?

A pandemic is an epidemic which is geographically spread over several continents following the appearance of a new virus.

In the case of influenza, this is most often a new subtype of the influenza A virus to which the population has not yet been exposed and against which it has not yet developed antibodies. The risk of its spread is therefore significant.

The different influenza viruses

The influenza virus includes 3 types:

  • A-type viruses, which can be the cause of pandemics (human and animal viruses)
  • B-type viruses, which are strictly human
  • C-type viruses, which appear only sporadically and generally give rise to benign symptoms.

Only the influenza A and B viruses are present in vaccines against seasonal influenza.

Some sub-types of Virus A


  • A(H1N1)

Influenza A(H1N1) viruses are of animal origin and have spread to the human population. They were the cause of the Spanish influenza in 1918 and a good number of seasonal influenzas.

The A(H1N1) pandemic influenza occurring in 2009 was initially called the “Mexican flu”, then “swine flu”, a reference to several fragments of the virus which are of swine influenza virus origin. This disease was caused by a new Influenza A virus of the sub-type H1N1 that was different from the A(H1N1) viruses that caused seasonal influenza, prior to the 2009 pandemic.

DID YOU KNOW? The symptoms of the A(H1N1) pandemic influenza were similar to those of seasonal flu. However, unlike seasonal influenza, the A(H1N1) virus of the pandemic flu affected proportionally more young people in whom the death rate was higher.  It appeared that people over the age of 65 had some degree of pre-existing immunity, having been exposed to a similar type of virus in the distant past.

  • A(H3N2)

The influenza A(H3N2) virus circulates in the human population during seasonal influenzas.

In 1968 the Hong Kong flu caused by the A(H3N2) virus became a pandemic and killed nearly one million people.

  • A(H5N1)

The influenza A(H5N1) virus is responsible for avian influenza, commonly called “bird flu”. These viral infections affect wild birds and domestic fowl and on rare occasions can spread to man or other animal species. The first case of human infection by A(H5N1) goes back to 1997 during an epidemic affecting poultry in Hong Kong. Since 2003 an increasing number of cases has been notified in various regions of Asia. Egypt is also a country which is regularly affected.

  • A(H7N9)

The influenza A (H7N9) virus is normally present in birds. Like the A(H5N1) virus, it is responsible for avian influenza. It was detected for the first time in man in China in March 2013. Most of the cases in man were the result of direct or indirect contact with poultry. Up to now the virus did not pass easily from person to person. The World Health Organization (WHO) considers that the transmission of the A(H7N9) virus between humans is very limited. The virus is, however, being monitored because mutations might render it more easily transmittable to man.

  • A(H5N8)

The influenza A (H5N8) virus is classed among the avian influenza viruses, the group of influenza viruses which infect birds. Identified for the first time in China in 2010, some outbreaks of the A(H5N8) virus have been detected in Europe, particularly in Germany and the Netherlands.

The A(H5N8) virus is very dangerous for birds but as yet no case has been observed in man. Scientists are monitoring it, as mutations could appear that would make it dangerous for human beings. Cases of infection in poultry were reported in November 2014 in Europe in the industrial rearing of ducks, turkeys and hens.

Sciensano coordinates a network of general practitioners and hospitals to ensure the permanent surveillance of influenza activity, of the intensity and severity of epidemics and the impact on the population. Sciensano is also the National Reference Centre (NRC) for influenza virus. The Belgian Official Medicines Control Laboratory (OMCL) of Sciensano, together with the European OMCL network, is responsible for the quality control of the influenza vaccine prior to marketing.

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