Seasonal flu (influenza)


The flu (= influenza) is a common infectious disease in wintertime. The risk of getting the flu and its complications can be reduced by a flu vaccination in autumn. It is recommended for certain risk groups and their contact persons.

Pathogen and transmission

The flu (= influenza) is caused by influenza A and influenza B viruses. For influenza type A, various subtypes exist, and for type B viruses there are two lineages: Victoria and Yamagata.

Influenza viruses are easily transmitted through respiratory droplets (sneezing, coughing), by direct contact (hands) or indirect contact (contaminated objects, e.g. door handles). Infected persons can transmit flu virus to others well before onset of any symptoms. In general, after infection it takes 1-3 days for the first symptoms to appear.


Typical symptoms of flu are a sudden onset of high fever (≥ 38° C), chills, coughing, sore throat, headaches, muscle and joint pain, and occasionally a runny nose, dizziness or loss of appetite.

Children quite often do also have nausea, vomiting and diarrhoea; in elderly persons fever may be absent. A flu can last up to two weeks.

An influenza infection might present with relatively mild symptoms without any complications, and it might therefore be easily mistaken for a common cold.

In contrast to other viral flu-like diseases, however, the "real" flu (influenza) can lead to sometimes serious complications. Throat, sinus and middle ear infections, pneumonia, myocarditis (inflammation of the heart) or neurological complications can be caused by influenza viruses or by so-called secondary bacterial infections.

The risk of severe complications is significantly increased for pregnant women, premature babies, people with certain chronic diseases or conditions, and for elderly persons. In rare cases, such complications can also affect healthy young adults.

Prevalence and frequency

Influenza viruses predominantly circulate in the colder months and cause an epidemic practically every year (flu wave). The intensity and severity of flu epidemics differs from one year to the next. In tropical regions, influenza occurs sporadically throughout the year.

In Switzerland, each year influenza leads to 112,000 to 275,000 medical consultations (according to the Sentinella monitoring system).

Due to its complications (see above), the flu is responsible for thousands of hospitalisations and a few hundred fatalities each year. People with an increased risk of flu complications (pregnant women, premature babies, elderly people and people with certain chronic diseases) are particularly prone to such outcomes.

Prevention: recommendations for the flu vaccination

Influenza vaccination is the easiest, most effective and cost-effective way prevention for protecting yourself and others from the flu and its complications. The ideal period for the flu shot runs from mid-October to mid-November.

The flu vaccination is recommended for persons with an increased risk of complications: people aged 65 and above, pregnant women, children born prematurely up to 2 years of age, and persons with chronic diseases. To protect these persons better, not only they, but everyone in regular contact with them should be vaccinated against the flu. The latter includes close family members, infant caregivers and health professionals.

The "National Flu Vaccination Day" takes place around the beginning of November. On this day, you can get your flu shot at participating MD’s offices without any prior appointment.

In some cantons, – usually with a medical prescription – it is also possible to get the flu shot in selected pharmacies.

Prevention: efficacy and side effects

Flu vaccine efficacy depends on age and general health status, but also on the match of vaccine strains with currently circulating influenza virus strains.

Influenza vaccination does not offer protection in any case: Studies estimate efficacy at 20 to 80 % depending on season and vaccinated individuals. In case of a weakened immune system, in elderly people and in patients with chronic conditions, vaccine efficacy is often reduced. In those cases where a flu occurs nevertheless, symptoms are often weaker and serious complications much less frequent.

The "trivalent" seasonal flu vaccine protects against two viral strains of influenza type A and one strain of influenza type B; the "quadrivalent" vaccine in addition also protects against a second type B strain.

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

In up to 25 % of cases, temporary pain or reddening can occur at the injection site after vaccination. Around 5 % of cases experience increased body temperature, muscular pain or a mild feeling of illness occur. Severe adverse reactions are not only extremely rare; they are also considerably rarer than the complications from the flu itself.

Last modification 01.02.2021

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