Seasonal influenza

Influenza is an acute viral infection that occurs worldwide. In the WHO European Region, annual influenza epidemics usually occur during autumn and winter and can infect up to 20% of the population.

Seasonal influenza can cause substantial mortality – a 2017 study found that worldwide up to 650 000 people die of influenza-associated respiratory disease each year, and up to 72 000 of these deaths occur in the European Region.

People most at risk of severe disease following influenza infection include people over 65, pregnant women, young children, immunocompromised people and people with chronic underlying medical conditions.

There are 2 main types of seasonal influenza viruses that cause illness in humans: type A and type B. Influenza A viruses are further classified into subtypes, while influenza B viruses are categorized in lineages. The most common circulating influenza viruses belong to subtype A(H1N1)pdm09 and A(H3N2), and to influenza B Yamagata and Victoria lineages.

Annual vaccination is the most effective way to prevent influenza infection and severe outcomes. Due to the constantly evolving nature of influenza viruses, WHO updates recommendations for the composition of the influenza vaccine twice a year: in February for the northern hemisphere and in September for the southern hemisphere. The recommendations are based on viruses detected and characterized by Member States through surveillance.

Ongoing surveillance of seasonal influenza around the Region is not only critical for the development of seasonal influenza vaccines; it also plays an important role in understanding the seasonality of influenza, assessing the impact and severity of annual epidemics, identifying groups at high risk of severe disease and mortality, determining the influenza disease burden, and preparing for pandemics.