Seasonal influenza

Influenza (flu) is a contagious disease with mostly respiratory symptoms caused by infection with an influenza virus. Every year during the winter season, seasonal influenza affects Europe and other parts of the northern hemisphere. It can cause epidemics of varying sizes. It is estimated that up to 20% of the population are infected with seasonal influenza each year.

What are the symptoms of the flu?

• fever or feverishness
• headache
• muscle pain
• general feeling of ill-health
• runny nose
• sore throat
• cough.

In addition to having a fever and respiratory symptoms, children with flu might also experience stomach-related symptoms like vomiting or diarrhoea. For adults with uncomplicated presentations, the systemic symptoms usually last for a few days, while a sore throat and runny nose may persist longer. Coughing, fatigue, and feeling unwell may continue for a few weeks. 

On average, it takes about two days for symptoms to appear after being exposed to the virus, but it can range from one to four days.

Key facts

Risk for people

Not everyone who gets infected will have symptoms. In fact, during a regular flu season, it has been estimated that around 65-85% of people with flu do not show any symptoms at all.

While complications can occur in anyone, the high-risk groups are:

• the elderly
• people of any age with chronic medical conditions (such as metabolic diseases, chronic lung conditions, heart disease, liver disease, blood conditions, morbid obesity, genetic conditions, chronic kidney diseases or treatments that suppress the immune function),
• pregnant women, and
• children under five years.

Influenza can cause serious problems, either from the virus itself or from additional bacterial infections. Pneumonia is a common complication, and sometimes it can be deadly. Influenza can also lead to other severe conditions like myocarditis (inflammation of the heart) and encephalitis (inflammation of the brain). The estimated overall mortality rate linked to influenza is 13.8 deaths per 100 000 people each year.

How it spreads

Influenza mainly spreads when someone coughs or sneezes, releasing droplets that can infect others nearby. It can also spread indirectly when people touch surfaces contaminated with respiratory secretions, like hands or tissues. If someone with the flu does not cover their mouth and nose while coughing or sneezing, people within a one-meter range may get infected. Additionally, there is some evidence suggesting that tiny infectious particles in the air (aerosols) may also contribute to the spread of influenza.

Vaccination and treatment

For most mild cases of seasonal influenza, treatment mainly focuses on reducing fever and relieving symptoms like cough and congestion. The main approach is to rest and stay at home and avoid spreading the virus to others. 

However, in more severe cases or for certain high-risk groups, antiviral medications can be prescribed. It's important to take these medications as soon as possible, ideally within the first 48 hours of illness onset. In hospitalised elderly patients, antivirals can reduce the risk of in-hospital death even if given within seven days from onset of symptoms.

Vaccination is the most effective way to prevent influenza, which is especially important for people at higher risk of serious complications: individuals with specific chronic medical conditions, pregnant women, children aged 6 months to 6 years, the elderly and healthcare workers.

Protective measures

In addition to vaccination, there are personal measures that people may take to reduce their risk of having flu:

• avoid close contact with sick people
• wash or clean your hands frequently
• avoid touching your eyes, nose or mouth.

If you are experiencing flu symptoms, there are also measures to avoid infecting others:

• maintain good respiratory hygiene and cough etiquette
• wash or clean your hands frequently
• surface and object cleaning
• increased ventilation
• stay home from work or school and limit contact with others 
• face masks: wearing a face mask should be considered in crowded settings where physical distancing is not possible.