Seasonal influenza (or “flu”) is most often caused by type A or B influenza viruses. Symptoms include sudden onset of fever, cough (usually dry), headache, muscle and joint pain, sore throat and a runny nose. The cough can be severe and can last 2 or more weeks. Most people recover from fever and other symptoms within a week without requiring medical attention. However, influenza can cause severe illness or death in high-risk groups.

Seasonal epidemics occur mainly during winter, from October to March in the northern hemisphere and April to September in the southern hemisphere. In tropical and subtropical countries, seasonal influenza can happen all year round.

The best way to avoid getting the flu is to get the flu vaccine every year. Influenza viruses evolve constantly, and twice a year WHO makes recommendations to update the vaccine compositions. For the 2016-2017 northern hemisphere influenza season, the vaccine formulation was updated in February 2016 to contain two type A viruses (H1N1 and H3N2), and a type B virus.

WHO recommends annual vaccination for high-risk groups including health care workers. People should ideally get vaccinated just before the influenza season begins for the most effective coverage, although getting vaccinated at any time during the influenza season can still help prevent flu infections.

People most at risk for severe seasonal influenza are:

• pregnant women at any stage of pregnancy
• children younger than 5 years
• people older than 65 years
• people with chronic medical conditions such as HIV/AIDS, asthma, heart and lung diseases and diabetes
• people with increased risk of exposure to influenza, which includes health care workers.