Seasonal Influenza

Influenza, commonly known as flu, is a highly contagious, acute, viral respiratory infection. Influenza viruses exhibit a high degree of variability. Depending on the extent of the genetic mutations from one year to another, the degree of protection in the population varies and influenza epidemics of variable intensity will occur. An influenza pandemic can occur when a totally new influenza virus against which the human population has no immunity emerges, usually from animals.


Symptoms of influenza

After an incubation period of one to four days(1), the first symptoms begin to appear with the abrupt onset of fever accompanied by malaise, headaches, muscle pain, sore throat and non-productive cough. Infection usually lasts one week.

Pneumonia is the most common complication and mostly occurs among young children, the elderly, and patients suffering from chronic diseases. Complications may lead to death, particularly among the most vulnerable groups.
Antiviral treatments are available and are effective in reducing both the intensity and duration of symptoms provided they are administered early in the course of the disease.
Influenza, a highly contageous disease

From the day before symptoms begin through approximately five days after illness onset(1)

Children can be infectious for more than 10 days after the onset of symptoms(1)

72% of the passengers became ill after travelling with one infected passenger, the apparent index case, on an airplane(2)


Epidemiology and vaccination against influenza

Influenza viruses are transmitted from one person to another through the inhalation of respiratory droplets or contact with respiratory secretions. According to World Health Organization (WHO) estimates, the number of influenza-related deaths ranges between 250,000 and 500,000 per year.(3) Several types of influenza vaccine are available.(4) Each year, the composition of influenza vaccines is adapted according to the dominant strains in circulation.



Influenza (Seasonal); WHO fact sheet N°211 April 2009: