Flu disease

Influenza virus belongs to the family Orthomyxoviridae. It is a lipid-enveloped, pleiomorphic (able to change its morphology) virus with helical nucleocapsid surrounded by peplomers. The outer envelope contains the surface antigens: neuraminidase (NA or N) and hemagglutinin (HA or H). Based on the antigenic characteristics of the internal proteins, matrix protein and nucleoprotein, it is possible to distinguish influenza viruses into 4 types:

• type A and type B which cause seasonal influenza epidemics
• type C, which is rarer and results in an infection that is usually asymptomatic or cold-like and of little epidemiological significance in humans
• type D, a newly identified type isolated only in pigs and cattle. Its possibilitỳ of infecting humans is unclear.

Influenza viruses have marked gene variabilitỳ with the ability to mutate the characteristics of the surface proteins HA and NA. On the basis of these mutations, viruses are classified into subtypes, each distinguished by a numerical suffix assigned in progressive typings. Type A influenza viruses have a greater ability to mutate. Variations are mainly due to spontaneous mutations (minor mutations or antigenic drift) of HA and NA antigens, which cause seasonal outbreaks and affect both type A and type B viruses. In the case of reassortments between human and animal viruses, a complete substitution of the HA and NA antigens (major mutation or antigenic shift) can occur, resulting in the appearance of a completely new viral strain compared to those previously circulating, leading to the development of influenza pandemics. This type of mutation occurs only in type A. It is precisely because of this characteristic that one does not develop permanent protection against influenza viruses and can get sick several times in a lifetime. Again for this reason, the vaccine must be reformulated annually with the most likely circulating strains and administered each year before the winter season.

Seasonal influenza is transmitted interhumanly through respiratory secretions or saliva, but also through contact with contaminated hands. A sick person begins to be able to infect others slightly in advance of the first symptoms and continues for 5-7 days.

The influenza virus is widespread throughout the world.
According to World Health Organization (WHO) estimates, seasonal influenza epidemics that occur annually during the winter season in temperate climate regions affect 5 to 15 percent of the world's population, or 350 million to 1 billion people. There are 3 to 5 million severe cases requiring hospitalization in those at risk (e.g., the elderly, people with chronic diseases). The impact of seasonal influenza in terms of mortalitỳ results in 250 to 500 thousand deaths each year, about 10% of severe cases.
The European Centre for Disease Prevention and Control (ECDC) estimates that up to 50 million symptomatic cases of influenza occur in Europe (European Union and European Economic Area, EU/EEA) between November and April each year, thus only a fraction of incident cases, and between 15,000 and 70,000 influenza-related deaths.

Influenza is an acute respiratory illness with an abrupt onset with fever usually exceeding 38 °C, accompanied by general symptoms such as headache, generalized malaise, asthenia, myalgia, sweating and chills, and respiratory symptoms such as cough, nasal congestion. Especially in children, gastrointestinal symptoms (nausea, vomiting, diarrhea) may also occur.
The incubation period is 1-4 days (average 2) and the disease usually lasts about 1-2 weeks and resolves with recovery.
In some cases, complications may occur: bacterial overinfection of the respiratory system (primary and secondary pneumonia) and ear (otitis, sinusitis, especially in children), complications of the cardiovascular system (myocarditis) and nervous system, as well as aggravation of preexisting diseases.

Diagnosis of influenza is essentially clinical. Laboratory tests on secretions from the respiratory tract or nasal cavities exist to detect the genome or viral antigens. Rapid tests with poor accuracy are also available. However, routine use of diagnostic tests is not recommended given the lack of relapse in clinical practice.

Treatment is symptomatic. Antipyretic and anti-inflammatory drugs are to be used for fever control and relief of patient discomfort. Given the significant use of self-prescription, people should be informed about the symptomatic nature of these therapies.
Paracetamol, ibuprofen, and diclofenac are drugs that can be used in adults and offer the advantage of good efficacy and low gastrolesiveness. In individuals at increased cardiovascular risk, the use of acetaminophen is recommended. For those alreadỳ on low-dose aspirin therapy, increasing the dose of acetylsalicylic acid to the minimum dose necessary to achieve the antipyretic and analgesic effect is an alternative. Acetylsalicylic acid, however, is not indicated for administration to children and young people under 18 years of age because of the risk of Reye's syndrome.
Antibiotic use is not recommended in flu syndrome without complications and in sore throat associated with flu syndrome unless proven to be caused by bacterial overinfection.

Influenza vaccine is the best preventive strategy to reduce the epidemiological, clinical and economic impact of influenza. Among the applicable strategies, the most widespread is the protection of at-risk population groups, particularly the elderly population. Despite extensive demonstrations of safety and efficacy, and despite the fact that the elderly are the group most at risk of developing complications after influenza, influenza coverage is not reaching the minimum recommended values.