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 contagious disease

  • The time from infection to illness, known as the incubation period, is about 2 days, but ranges from one to four days.(1)
  • Children can be infectious for more than 10 days after the onset of symptoms(1)

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 290,000 to 650,000 per year.(1)
  • Several forms of influenza vaccine are available.(1)
  • Each year, the composition of influenza vaccines is adapted according to the dominant strains in circulation.


1 - WHO. Influenza (Seasonal) fact sheet. Reviewed January 2018; accessed January 2018.

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