What is flu?

More and more people fall ill with flu, especially in the colder months of the year, which is why it is also called seasonal flu. It is an acute infectious respiratory disease caused by certain influenza viruses. Flu usually begins suddenly with fever, a dry cough, a sore throat, muscle pain and headache. The symptoms usually affect the whole body as well as the respiratory tract. Those affected suddenly feel unwell.

Other illnesses, such as a common cold, can cause similar symptoms, which is why a cold is also called a “flu-like infection”. A cold, however, is caused by different viruses from those that cause flu.

Leading countries in medicine agree that vaccination is the best protection against the influenza virus. The majority of people receiving treatment in Switzerland for complications after flu are unvaccinated. Flu vaccines are inactivated vaccines that contain only inactive parts of the virus. The human immune system reacts with an immune response in which, among other things, antibodies are formed. Some people show a noticeable reaction to the vaccination, both locally around the injection site and systemically, in the form of malaise, headache and aching limbs, nausea or fever. This is rare with the flu vaccine, however, and lasts only a day or two.

What are the symptoms of flu?

Everyone knows the symptoms of a cold, such as a runny nose, cough and scratchy throat. Flu can show very similar symptoms, but they are usually more pronounced.

Typical flu symptoms:

  • Sudden onset of fever
  • A dry, hacking cough or a sore throat
  • Muscle pain and aching limbs
  • Severe headache.

In addition, general weakness, sweating and a runny nose may occur.

In children in particular, flu can also lead to gastrointestinal problems such as nausea, vomiting and diarrhoea.

The clinical picture of flu is very varied. Not all infected people have typical flu symptoms. Around one third of those infected actually have a fever and typical symptoms, another third have only mild cold symptoms without fever, and the final third have no symptoms at all.

In older people or people with weakened immune systems in particular, high fever and pronounced signs of illness are often absent. Older people may, however, also show symptoms such as confusion and disorientation.

Flu vaccination

True viral influenza (flu) is a serious illness, far more than a simple cold. People with chronic illnesses, those over 60 and pregnant women should be vaccinated in particular, as these groups have a higher risk of developing the disease with serious consequences. The vaccination recommendation applies equally to medical staff, since frequent close contact with patients generally increases the risk of infection. There is also a risk that they will pass the influenza virus on to their patients, possibly without even noticing.

Flu vaccination should be given every year, ideally between October and mid-December. After vaccination, the body needs 10 to 14 days to build adequate protection against infection. A later vaccination at the beginning of the year is usually still worthwhile, especially if the flu epidemic has not yet begun or has only just started.

Effectiveness and action

Protection against flu is achieved within two weeks of vaccination: vaccination should be repeated every year in autumn to protect against the following winter epidemic. Only one injection is needed.

The composition of the vaccine is updated every year in line with the recommendations of the World Health Organization (WHO) and takes into account the viruses most likely to circulate during the winter. Flu is unpredictable, and the effectiveness of the vaccine varies from year to year. The vaccine is all the more effective when the influenza virus types selected for its development are close to the strains actually circulating during the seasonal epidemic.

Many vaccinated people believe they have had flu despite the vaccination. There can be two reasons for this:

  • it really is flu, and the person did not respond to the vaccine, because its effectiveness is below 100%;
  • it is a different virus (a viral infection) whose symptoms resemble those of flu, rather than true flu.

The vaccine cannot cause flu under any circumstances, as it contains only an inactivated fraction of the virus.