Seasonal flu is an acute respiratory infection caused by influenza viruses circulating around the world

Seasonal flu is an acute respiratory infection caused by influenza viruses circulating around the world.

There are 4 types of seasonal flu viruses - types A , B , C and D. Influenza A and B viruses circulate and cause seasonal epidemics of the disease.

Influenza A viruses are divided into subtypes in accordance with the combinations of hemagglutinin (HA) and neuraminidase (NA) , proteins on the surface of the virus. Influenza viruses of the subtypes A (H1N1) and A (H3N2) are currently circulating among people . A (H1N1) is also referred to as A (H1N1) pdm09 because it caused a pandemic in 2009 and subsequently replaced the seasonal influenza A (H1N1) virus that circulated until 2009. It is known that only influenza A viruses caused pandemics.

Influenza viruses are not divided into subtypes, but can be divided into lines. Currently circulating influenza B viruses belong to the B / Yamagata and B / Victoria lines.

Influenza virus C is detected less frequently and usually leads to mild infections. Therefore, it does not represent a public health problem.

Group D viruses mainly infect cattle. According to reports, they do not infect people and do not cause their diseases.

Signs and symptoms
Seasonal flu is characterized by sudden onset of fever, cough (usually dry), headache, muscle and joint pain, severe malaise, sore throat and runny nose. Coughing can be severe and last 2 weeks or more. In most people, the temperature returns to normal and the symptoms disappear within a week without any medical attention. But flu can lead to severe illness and death, especially in people at high risk (see below).

The disease can be mild or severe, and even fatal. Hospitalizations and deaths occur mainly in high-risk groups. It is estimated that annual influenza epidemics lead to 3-5 million cases of severe illness and 290,000 to 650,000 deaths from respiratory diseases.

In industrialized countries, the majority of influenza-related deaths occur in people 65 years and older (1).
Epidemics will result in high levels of absence from work / faculty and productivity losses.
During peak periods, the incidence of clinics and hospitals may be crowded.
The consequences of seasonal respiratory disease epidemics in developing countries aren't documented, however analysis results show that ninety nine of deaths of kids underneath five years old-time with influenza-related
lower respiratory infections occur in developing countries (2).

People get the flu at any age, but there are populations at higher risk .
Pregnant women, children under the age of 59 months, the elderly, people with chronic health problems (such as chronic diseases of the heart, lungs and kidneys, metabolic disorders, disorders of neurological development, liver disease and blood tests) and immunocompromised people (as a result of HIV / AIDS, chemotherapy or steroid treatment, and also due to malignant neoplasms).
Health care workers are at high risk of becoming infected with the flu virus during patient contact and can further transmit the infection, especially to people at risk.

Transmission of seasonal flu infection occurs easily and quickly, especially in areas of large concentrations of people, including schools and boarding schools. When an infected person coughs or sneezes, small droplets containing the virus (infectious droplets) get into the air and can spread up to one meter away and infect people in the vicinity who inhale them. Infection can also be transmitted through hands contaminated with influenza viruses. To prevent coughing, you should cover your mouth and nose with a tissue and wash your hands regularly.

In temperate regions, seasonal epidemics occur mainly in the winter season, while in tropical regions influenza viruses circulate year-round, leading to less regular epidemics. Seasonal epidemics and the burden of disease

The time period from the moment of infection to the development of the disease, known as the incubation period , lasts about 2 days, but can vary from 1 to 4 days.

In most cases, human influenza is clinically diagnosed. However, during periods of low activity of influenza viruses and in the absence of epidemics, infection caused by other respiratory viruses, such as rhinovirus, respiratory syncytial virus, parainfluenza virus and adenovirus, can also occur as a flu-like illness, which makes it difficult to clinically differentiate influenza from other pathogens.

To make a final diagnosis, it is necessary to collect proper respiratory samples and perform a laboratory diagnostic test. The first critical step for laboratory detection of influenza viral infections is proper collection, storage and transportation of respiratory specimens. Usually, laboratory confirmation of influenza viruses in secretions from the throat, nose, and nasopharynx, or in aspirates or washes from the trachea is performed by direct detection of antigens, isolation of viruses, or detection of influenza-specific RNA using reverse transcriptase polymerase chain reaction (RT-PCR). There are a number of lab guidelines published and updated by WHO (3).

Rapid diagnostic tests are used in flu clinics, but compared with RT-PCR methods they have low sensitivity, and the reliability of their results largely depends on the conditions in which they are used.

Patients who are not in high-risk groups should receive symptomatic treatment. If symptoms are present, they are advised to stay at home in order to minimize the risk of infection of other people in the community. The treatment is aimed at alleviating the symptoms of the flu, such as high fever. Patients should monitor their condition and in case of deterioration seek medical help. If it is known that patients are at high risk of severe illness or complications (see above), in addition to symptomatic treatment, they should receive antiviral drugs as soon as possible.

Patients with severe or progressive clinical illness associated with suspected or confirmed influenza viral infection (for example, with the clinical syndromes of pneumonia, sepsis or exacerbation of concomitant chronic diseases) should receive antiviral drugs as soon as possible.

To obtain the maximum therapeutic effect, neuraminidase inhibitors (eg, oseltamivir) should be administered as soon as possible (ideally within 48 hours after the onset of symptoms). For patients at a later stage of the disease should also include medication.
Treatment is recommended for at least 5 days, but can be prolonged until satisfactory clinical results are obtained.
The use of corticosteroids should be considered only if there are other indications (such as asthma and other specific health problems), since it is associated with longer elimination of viruses from the body and weakening of the immune system, which leads to bacterial or fungal superinfection.
All currently circulating influenza viruses are resistant to antiviral drugs of the adamantan class (such as amantadine and rimantadine), so these drugs are not recommended to be used as monotherapy.
The WHO GISRS monitors antiviral drug resistance among circulating influenza viruses in order to provide timely guidance on the use of antiviral drugs for clinical management and potential chemoprophylaxis.
The most effective way to prevent the disease is vaccination . For more than 60 years, safe and effective vaccines have been available and used. Some time after vaccination, immunity weakens, so annual vaccination is recommended to protect against flu. The most widely used in the world injection inactivated influenza vaccines.

Among healthy adults, the anti-influenza vaccine provides protection even if circulating viruses do not match exactly the vaccine viruses. However, for the elderly, influenza vaccination may be less effective in preventing disease, but it reduces its severity and reduces the likelihood of complications and death. Vaccination is especially important for people at high risk of developing complications, as well as for people living with people at high risk or caring for them.

WHO recommends annual vaccination for the following populations:

pregnant women at any stage of pregnancy
children aged 6 months to 5 years
the elderly (over 65)
people with chronic health problems
health workers.
The effectiveness of the influenza vaccine depends on how much the circulating viruses coincide with the viruses contained in the vaccine. Due to the constantly changing nature of influenza viruses, the WHO Global Influenza Surveillance and Response System (GISRS) - a system of national influenza centers and WHO collaborating centers around the world - continuously monitors influenza viruses circulating in humans and updates them twice a year composition of influenza vaccines.

Over the years, WHO has been updating its recommendations on the composition of the vaccine (trivalent), targeting the 3 most common circulating types of virus (two subtypes of influenza A virus and one subtype of influenza B virus). Beginning with the 2013-2014 influenza season in the northern hemisphere, a fourth component is recommended to help develop a quadrivalent vaccine. The quadrivalent vaccines, in addition to the viruses in the trivalent vaccine, include the second type B influenza virus, and are expected to provide greater protection against infections caused by the influenza B virus. Many inactivated and recombinant influenza vaccines are available in injectable form. A live, attenuated influenza vaccine is available in the form of a nasal spray.

Pre-exposure and post-exposure prophylaxis with antiviral drugs is possible, but its effectiveness depends on a number of factors, such as individual characteristics, type of exposure, and risk associated with exposure.

In addition to vaccination and antiviral treatment, public health measures include individual protection measures , such as:

regular washing and proper drying of hands;
proper respiratory hygiene - covering the mouth and nose when coughing and sneezing with napkins, with their subsequent proper removal;
timely self-isolation of people who felt unwell, with fever and other flu symptoms;
preventing close contact with sick people;
preventing contact with eyes, nose and mouth.

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