"Seasonal influenza" refers to the influenza outbreaks that usually occur each winter in both the northern and the southern hemispheres, and year round in the tropics. Outbreaks occur each year because the influenza virus undergoes constant but relatively minor genetic changes called "antigenic drift". Antigenic drift prevents people from developing lifelong immunity. People who catch the flu one year will develop immunity to that year's flu strain, but will probably be susceptible to next year's strain since its genetic structure will have changed slightly.

In general, seasonal influenza is more severe in very young children, people over 65 years old, and those with underlying health conditions.

During fall/autumn in each hemisphere, WHO assesses which influenza strains have been active during the winter in the other hemisphere and recommends the influenza subtypes to be included in this year's vaccine. Most years the seasonal flu vaccine contains the correct influenza subtypes and effectively prevents influenza. Influenza vaccines cannot cause influenza.

Download our flu fact sheet for more information on preventing the spread of flu in your organisation.

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