Seasonal Flu Overview

Updated: March 2017

What is seasonal flu?

"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.

Who is most at risk from seasonal flu?

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

Prevention of seasonal flu

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.

How does seasonal flu differ from pandemic flu?

Pandemic flu is a "new" type of influenza virus that hasn't circulated in humans, hence people have little or no immunity against it. The virus can cause significant illness or death. As the virus can spread easily from person to person, it spreads globally - this is known as a "pandemic." The severity of the pandemic depends on the qualities of the virus. Some cause more severe illness and are more infectious than others.

Pandemic flus eventually become seasonal flus as they continue to circulate in the human population, and eventually people develop immunity to it. The strain that caused the influenza pandemic in 2009/2010 (influenza A/H1N1 pdm 09) is now a seasonal flu.

It is unknown exactly how a pandemic flu arises, and no one knows when another pandemic will occur, however the experts advise that another pandemic is inevitable. The avian influenza A strains H5N1 and H7N9 have pandemic potential and are being watched carefully in case they develop the ability to spread easily from one person to another.