Seasonal flu (influenza) (2024)

Flu (influenza) is an infectious disease that is common during the winter. You can reduce the risk of catching flu and of complications by getting a flu vaccine in the autumn. The flu jab is recommended for certain at-risk groups and their close contacts.

Flu viruses and transmission

Flu is transmitted by influenza A and influenza B viruses. There are various sub-types of influenza A viruses, while for B viruses there are two lineages: Victoria und Yamagata. The flu virus is highly transmissible. Transmission can occur through direct contact with an infected person (sneezing, coughing, or hands), particularly in enclosed spaces, or through indirect contact (e.g. via objects or door handles).

People who have been infected with flu viruses can transmit them to others, even if they do not (yet) feel ill. It takes around one to three days for symptoms to appear.

Symptoms

Typical symptoms of flu are a sudden high temperature (>38 °C), chills, coughing, a sore throat or difficulty swallowing, a headache, joint and muscle pain, but also a cold, dizziness and a loss of appetite. Children may also experience nausea, vomiting and diarrhoea, while older adults may not have a high temperature. Flu can last up to two weeks. Flu can also be relatively mild and without complications and is thus often confused with the common cold.

However, unlike other cold viruses, ‘real‘ flu (influenza) can lead to many complications. For example, the influenza viruses themselves or a secondary bacterial infection can result in throat, sinus and middle ear infections, pneumonia, myocarditis, or neurological complications.

The risk of serious complications is significantly increased in pregnant women, premature babies, people with certain chronic diseases, and older people. In rare cases, young otherwise healthy people may experience serious complications.

Spread and prevalence

Influenza viruses mainly spread during the cold winter months, causing a flu epidemic virtually every winter. The 2020/21 flu season was an exception. Due to the measures in place to contain the COVID-19 pandemic, there was no flu epidemic in the winter of 2020/21 and cases of flu only occurred sporadically. Although the intensity and severity of flu epidemics vary from year to year, the fact that there was no flu epidemic is exceptional, but was also observed in other countries in the temperate zones. Flu occurs sporadically all year round in the Tropics.

In Switzerland, flu usually results in 112,000 to 275,000 doctor’s appointments (according to the Sentinella reporting system). On account of the complications (see above), it also leads to several thousand hospitalisations and several hundred deaths. This mainly affects people with an increased risk of flu complications (pregnant women, premature babies, older adults and people with certain chronic diseases).

Prevention: flu jab recommendations

The flu vaccine is the simplest, most effective and cheapest way of preventing yourself and others from flu and its complications. The flu vaccination season starts in mid-October and lasts until the start of the flu season.

The flu vaccine is recommended for those with an increased risk of complications: people aged 65 or over, pregnant women, children born prematurely under the age of two, and people with chronic diseases. To better protect these people, not only they, but also anyone with whom they come into regular contact, should be vaccinated. This includes relatives, those caring for infants and healthcare professionals.

The Swiss National Flu Vaccination Day, when you can usually get a flu jab without an appointment, takes place every year in November. In most cantons it is also possible to get a vaccine in certain pharmacies that offer vaccination.

Efficacy and side effects

The efficacy of the vaccine depends on people’s age and state of health, and on the influenza virus strains that are currently circulating.

The flu vaccination does not offer complete protection: studies estimate that efficacy is between 20% and 80% depending on the season and the individual receiving the vaccine. The effectiveness is reduced for people with a weakened immune system, older adults and people with chronic diseases. However, if these people do catch flu, their symptoms are often milder and they are less likely to suffer serious complications.

The ‘quadrivalent‘ seasonal flu vaccines protect against two type-A influenza virus strains and two type-B strains.

The flu vaccine does not protect you against common cold viruses, or against COVID-19.

Temporary pain or redness at the injection site occurs in up to 25% of people. A temporary high temperature, muscle pain or feeling slightly unwell occurs in around 5% of vaccinated people.

With the nasal spray influenza vaccine, which contains attenuated, replicable vaccine viruses, temporarily decreased appetite, headache, nasal congestion or runny nose and malaise may occur.

Severe adverse effects are not only extremely rare for both vaccine types, they are also many times less likely than experiencing complications linked to flu.