Why does the flu have a season

Seasonal flu (influenza)

Pathogen and transmission

The flu is transmitted by influenza A and influenza B viruses. There are various subtypes of type A viruses and two strain lines of B viruses: Victoria and Yamagata. The flu virus can easily be transmitted through direct contact (sneezing, coughing or hands), especially in closed rooms, or through indirect contact (e.g. via objects, door handles).

People who have become infected with the flu virus can pass them on to others, even if they do not (yet) feel sick. It takes about one to three days for the disease to begin.

Clinical picture

Typical symptoms of the flu are sudden high fever (> 38 ° C), chills, cough, sore throat and sore throat, headache, pain in muscles and joints, but also runny nose, dizziness and loss of appetite. Children may also experience nausea, vomiting and diarrhea, and older people may not have a fever. The flu can last up to two weeks. Flu can be relatively mild and uncomplicated and is then often mistaken for a cold.

In contrast to other viral colds, the "real" flu can lead to numerous complications. Sore throat, sinus and middle ear infections, pneumonia or heart muscle inflammation or neurological complications can result from the influenza virus itself or from a secondary bacterial infection.

The risk of serious complications is significantly increased in pregnant women, premature babies, people with certain chronic diseases and in the elderly. In rare cases, these can also affect younger, previously healthy people.

Distribution and frequency

Influenza viruses mainly circulate in the cold season and cause an epidemic (flu wave) practically every winter. The intensity and severity of the flu wave vary from year to year. In the tropics, the flu occurs sporadically all year round.

In Switzerland, the flu leads to 112,000 to 275,000 doctor consultations every year (according to the Sentinella monitoring system). Due to disease complications (see above), there are also several thousand hospitalizations and several hundred deaths. It mainly affects people with an increased risk of flu complications (pregnant women, premature babies, the elderly and people with certain chronic diseases).

Prevention: flu vaccination recommendations

The flu vaccination is the simplest, most effective and cheapest way of preventing yourself and your fellow human beings from getting flu and its complications. The flu vaccination season begins in mid-October and lasts until the beginning of the flu wave.

The flu vaccination is recommended for people with an increased risk of complications: People aged 65 and over, pregnant women, premature children up to two years of age and people with chronic illnesses. In order to better protect these people, not only themselves but also everyone who is in regular, close contact with them should be vaccinated against the flu. These include close relatives, infant carers and health professionals.

At the beginning of November, the national flu vaccination day takes place, during which you can usually be vaccinated against the flu without prior notification. In some cantons it is also possible in autumn to be vaccinated directly in pharmacies with a corresponding vaccination offer.

Effectiveness and side effects

The effectiveness of the vaccination depends on the age and state of health as well as the currently circulating influenza virus strains.

The flu vaccination does not protect in every case: studies estimate the effectiveness depending on the season and vaccinated people at 20 to 80%. The effectiveness is reduced in the case of a weakened immune system, in the elderly and people with chronic diseases. If the flu does occur, the symptoms are often reduced and serious complications are less common.

The “trivalent” seasonal flu vaccines protect against two virus strains of influenza type A and one influenza type B strain, the “quadrivalent” vaccines additionally against a second type B strain.

The flu vaccination does not protect against the common, but usually harmless, cold viruses.

In up to 25% of the patients, pain or reddening may occur briefly after the vaccination at the injection site. Around 5% experience a temporarily elevated temperature, muscle pain or a slight feeling of illness. Serious adverse effects are not only extremely rare, they are also many times less common than the complications in the case of influenza.