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Istituto Superiore di Sanità
EpiCentro - Epidemiology for public health
Epidemiology for public health - ISS

About seasonal influenza (flu)

Influenza is a public health problem with significant epidemiological, clinical and economic implications. This is due to a number of factors: ubiquity and infectivity of the disease, antigenic variability of the viruses, seasonal epidemic (and periodically pandemic) outbreaks, possible severe complications in certain groups of people (children, elderly, people with comorbidities and chronic diseases), cost of managing complications and social costs (e.g. lost working days, loss of productivity).


Influenza is an acute respiratory disease caused by influenza viruses. It is a seasonal disease that, in the western hemisphere, occurs during the winter months. Influenza virus was first isolated from humans in 1933, in the United Kingdom (influenza viruses had previously been isolated from pigs and poultry). Four different types have since been identified, all of them belonging to the Orthomyxoviridae family: types A and B, which are responsible for the typical symptoms of seasonal influenza; type C, of little clinical relevance (generally asymptomatic); type D, whose potential to infect humans is still unknown. Influenza A virus is further divided into subtypes, based on molecular differences in the haemagglutinin (HA) and neuraminidase (NA) surface glycoproteins.


Influenza viruses have a strong tendency to mutate. By undergoing antigenic variation in their HA and NA glycoproteins, they can elude the immune response acquired from previous infections, and therefore spread widely and quickly within a population that has largely become immunologically susceptible.


These molecular variations should be taken into account when developing vaccines, the composition of which should be updated every year. Surveillance activities are therefore crucial to help select the strains to include, based on the degree of epidemiological and serological difference between such strains and the viruses that circulated in previous seasons.


Publication date: 28 January 2021