Influenza is a seasonal infectious disease caused by influenza viruses.
Three different types of viruses are known, differing from each other based on the proteins present on their surface: types A and B which cause the classic flu, and type C which causes an infection that is usually asymptomatic or has symptoms similar to a common cold. An important feature of flu viruses is that they are able to change the characteristics of their surface proteins. It is precisely because of this property that flu epidemics occur every year and that, unlike many other diseases, even if a person contracts it, there is no protection against subsequent infections (immunity). For this reason the flu vaccine must be re-prepared annually with the strains in circulation, and so, administered every year.
The infection is spread from the patient to the healthy susceptible person through the droplets of saliva produced and emitted during coughing, sneezing and phonation (simply speaking); in this case we speak of direct transmission. It can also be spread through contact with objects or surfaces (even hands) on which saliva droplets or the secretions from sick people are present.
A person with the flu can transmit the disease from 24 hours prior to the onset of symptoms onwards; the virus usually stops being present in nasal secretions within 7 days of the onset of the disease. In young children and people with a compromised immune system, however, the virus can persist for longer. Contracting flu is more likely when attending crowded places with frequent cause for contact, which explains why the disease is more widespread in winter.
Sintomi e complicanze
Influenza is usually self-limiting with a benign clinical outcome. In some groups of people, such as children and the elderly, it can lead to potentially serious complications. The disease usually has a short incubation period, 1-3 days, and is characterised by a sudden onset of symptoms such as:
- High fever accompanied by shivering
- General malaise
- Muscular pain
- Joint pain
- Lack of appetite
These are all followed by and accompanied by symptoms involving the respiratory tract:
- Sore throat
- Nasal congestion
In children, the flu can also affect the gastrointestinal tract, causing nausea, vomiting and diarrhoea. Generally, flu patients recover with no consequences although in some cases the disease causes complications that can be fatal. Children, the elderly and people with chronic conditions are more at risk of developing complications. The flu becomes more complicated when the infection no longer remains confined to the upper respiratory tract, but spreads to the bronchi and/or lungs.
Complications of influenza include bronchitis and pneumonia; the latter is particularly alarming for those who already suffer from chronic lung or heart disease, so much so that it increases hospitalisation rates by 3-5 times. Other possible complications are sinusitis and otitis (especially in children). Furthermore, in at-risk patients (for example cardiopaths and bronchopneumopathics), it can lead to a rapid worsening of the already present disease and have a fatal outcome.
Impact on the population
Influenza is a seasonal disease that occurs in our country especially in the winter period with a peak between March and December. Sporadic infections can also occur outside the normal flu season, although in the summer months the incidence is very low. It is a frequent cause of medical consultation and hospitalisation and is the main cause of absence from work (10% of all absences from work) and from school, thus absenteeism increases by 56% during the flu season causing the loss of 500,000 working days during peak flu season. In Italy the average duration of absence from work is 4.8 days, and each case of influenza costs a total of 330 euro. The disease is also an important public health problem due to the number of cases occurring every season, to a greater or lesser extent depending on the transmissibility of the flu virus in circulation. A significant problem stems from the fact that it can manifest with symptoms very similar to those of other diseases and this has two effects: on the one hand improper hospitalisation for syndromes that are not influenza, and on the other, the signs of the disease can be underestimated in terms of morbidity and mortality.
The disease can affect every age group but some people are more at risk (children, the elderly, chronic patients). Cases of influenza vary from year to year but it is estimated that the disease affects 10-20% of the general population. The incidence of the disease varies from 5% to 50% of the general population during pandemics. The age group most affected is children between 0 and 14 years, with an incidence of 15%, while the highest mortality is observed in people over 60 years of age, where 90-94% of deaths occur. It has in fact been shown that over-65s have a 70% greater risk of developing complications.
Sources / Bibliography
- INFLUNET: sistema di sorveglianza sentinella delle sindromi influenzali in Italia Rapporto sulla stagione influenzale 2010-2011
- Medico e Bambino Prevenzione e controllo dell’influenza: raccomandazioni per la stagione 2010-2011 C. Rizzo, M. C. Rota 2010;29:504-507
- Igiene Epidemiologia Sanità Pubblica 2011; 269-287 C. Signorelli
- Manuale MSD Influenza www.msditalia.it/altre/manuale/sez13/1621383.htm
- Ministero della Salute
- Istituto Superiore di Sanità