Flu is a highly contagious viral disease. The name “influenza” originated in 15th century Italy, when epidemics were attributed to the “influence of the stars.” The first pandemic (worldwide epidemic) that clearly corresponded to the description of influenza was recorded in 1580. Four influenza pandemics were recorded in the 19th century, and three occurred in the 20th century. The Spanish flu pandemic of 1918-1919 killed about 21 million people worldwide.
In 1933, British virologists Smith, Andrews and Laidlaw isolated the influenza A virus, three years later Francis isolated the influenza B virus. In 1947, Taylor first isolated the influenza C virus.
In 1936, Burnet discovered that the influenza virus could be grown (cultured) in embryonic chicken eggs. This led to the possibility of studying the characteristics of the influenza virus, and subsequently to the development of vaccines. The protective effectiveness of inactivated vaccines was established already in the 1950s.
Prevalence and seasonality
The incidence of influenza is widespread throughout the world. Seasonal epidemics occur every year - in the northern hemisphere usually from November to March (may start earlier and end later), in the southern hemisphere from June to October. This is due to the onset of the cold season of the year.
In the northern hemisphere, peak activity is most often recorded in January, February or March.
During the influenza season, in total, from 5 to 15% of the world's population is infected, and from 3 to 5 million cases of severe influenza are registered. According to WHO estimates, the number of deaths from influenza in the world is 250,000 - 500,000 cases per year.
In case of infection, you must immediately make an appointment with a therapist.
Treatment
Patients not included in high-risk groups should receive symptomatic treatment. If they have symptoms, they are advised to stay home to minimize the risk of infecting others in the community. Treatment is aimed at easing flu symptoms, such as fever. Patients should monitor their condition and seek medical attention if it worsens. If patients are known to be at high risk of developing severe disease or complications (see above), they should receive antiviral drugs as soon as possible in addition to symptomatic treatment.
Patients with severe or progressive clinical illness associated with suspected or confirmed influenza virus infection (eg, clinical syndromes of pneumonia, sepsis, or exacerbation of concomitant chronic diseases) should receive antiviral drugs as soon as possible.
- To obtain maximum therapeutic effect, neuraminidase inhibitors (eg, oseltamivir) should be prescribed as soon as possible (ideally within 48 hours of symptom onset). For patients at a later stage of the disease, medication should also be considered.
- Treatment is recommended for at least 5 days, but can be extended until satisfactory clinical results are obtained.
- The use of corticosteroids should only be considered for other indications (such as asthma and other specific health conditions) because they are associated with longer clearance of viruses from the body and weakened immunity, leading to bacterial or fungal superinfection.
- All currently circulating influenza viruses are resistant to the adamantane class of antivirals (such as amantadine and rimantadine), so these drugs are not recommended for use as monotherapy.
The WHO GISRS monitors antiviral resistance among circulating influenza viruses to provide timely guidance on the use of antiviral drugs for clinical management and potential chemoprevention.
How is the flu transmitted?
Influenza is primarily spread from person to person through large droplets of phlegm or mucus (particles larger than 5 microns in diameter) produced when infected individuals sneeze and cough. These large droplets can settle on the surface of the mucous membranes of the upper respiratory tract of other people nearby (within 1 meter).
Transmission can occur through direct or indirect contact with respiratory secretions; for example, a person can become infected with influenza by touching surfaces or objects contaminated with influenza viruses (such as droplets or a used handkerchief) and then touching their mouth, eyes, or nose. The influenza virus in the external environment can remain viable on hard surfaces for up to 24 hours.
Pathogen
There are 4 types of seasonal influenza viruses - types A, B, C and D. Influenza A and B viruses circulate and cause seasonal epidemics of illness.
Influenza A viruses are divided into subtypes according to combinations of hemagglutinin (HA) and neuraminidase (NA), proteins on the surface of the virus. Currently, influenza viruses of subtypes A(H1N1) and A(H3N2) are circulating among people. A(H1N1) is also referred to as A(H1N1)pdm09 because it caused the 2009 pandemic and subsequently replaced the seasonal influenza A(H1N1) virus that circulated before 2009. Only influenza A viruses are known to cause pandemics.
Influenza B viruses are not divided into subtypes, but can be divided into lineages. Currently circulating influenza B viruses belong to the B/Yamagata and B/Victoria lineages.
Influenza C virus is less common and usually causes mild infections. Therefore, it does not pose a public health problem.
Group D viruses mainly infect cattle. According to available data, they do not infect people or cause disease in them.
Contagious period
It is possible to transmit influenza to other people before clinical symptoms appear. Most healthy adults can infect others from 1 day before symptoms appear until 5 to 7 days after the onset of illness.
Some people, especially children and people with weakened immune systems, can infect others for even longer periods of time—10 days or more. The degree of contagiousness decreases as symptoms of the disease decrease, and once signs of the disease have disappeared, patients are no longer considered contagious.
After airborne transmission and entry into the human respiratory tract, the virus attaches to the epithelial cells of the trachea and bronchi and penetrates them. The virus multiplies in the host cell, which leads to its destruction. The respiratory tract is a source of influenza virus for 5-10 days.
Clinical signs
The incubation period for influenza is usually 2 days, but can range from 1 to 4 days. Disease severity
depends on the presence of immunity as a result of past encounters of the body with various variants of influenza viruses. Generally
only approximately 50% of infected individuals will develop classic clinical symptoms of influenza. "Classic" flu
characterized by sudden fever, muscle pain, sore throat, cough without phlegm (nonproductive cough)
and headache.
Typically, body temperature rises to 38.3–38.9 °C and is accompanied by extreme weakness. The onset of fever is so abrupt that the patient can indicate the exact time. Back muscles often hurt. Cough develops as a result of the death of tracheal epithelial cells. Symptoms may also include a runny nose, burning behind the breastbone, eye pain and sensitivity to light.
General symptoms and fever usually last 2 to 3 days, rarely more than 5 days. These symptoms can be reduced by taking aspirin or paracetamol. Aspirin should not be given to infants, children and adolescents because After influenza, they are at risk of developing Reye's syndrome (acute hepatic encephalopathy).
Recovery from the flu is usually quick, but some patients may experience prolonged asthenia (weakness and decreased activity) and depression for several weeks.
Complications
The most common complication of influenza is pneumonia, especially secondary bacterial pneumonia (caused, for example, by Streptococcus pneumoniae, Haemophilus influenzae or Staphylococcus aureus). Primary viral influenza pneumonia is a rare complication with high mortality.
Reye's syndrome (acute hepatic encephalopathy) is a complication that occurs almost exclusively in children and adolescents taking aspirin and is primarily associated with influenza B (or chickenpox). It manifests itself as severe vomiting, confusion, which can turn into a coma due to cerebral edema.
Other complications include myocarditis (inflammation of the heart muscle) and worsening of existing chronic diseases, especially chronic lung diseases. Death is recorded at a rate of 0.5-1 per 1000 cases of influenza. Most deaths occur in people aged 65 and older.
International and foreign data
Long-term surveillance of influenza epidemics in the United States showed that during the period 1976-1990, there were approximately 19,000 influenza-related cardiac and pulmonary deaths each influenza season. In the period 1990-1999. Approximately 36,000 deaths have already been recorded. People aged 65 years and older account for more than 90% of deaths associated with influenza and pneumonia. The rise in flu-related deaths in the United States is partly due to an increase in the elderly population. In addition, influenza seasons with influenza A (H3N2) virus predominance are associated with higher mortality.
The risk of developing complications and hospitalization from the flu is higher among people age 65 and older, young children, and people of any age with certain underlying health conditions. In the United States, on average, more than 200,000 hospitalizations are associated with influenza during the year, more than 57% of them in people under 65 years of age.
More hospitalizations were recorded during years of predominant A(H3N2) virus circulation. In nursing homes, morbidity rates can be as high as 60%, with mortality rates as high as 30%.
Among children 0–4 years of age, hospitalization rates ranged from 100 per 100 thousand in healthy children to 500 per 100 thousand in children with comorbidities.
Hospitalization rates for children 24 months and younger are comparable to those for those 65 years and older.
Children 24-59 months of age are less likely to be hospitalized for influenza than younger children, but are among the group that has the highest rates of outpatient care use, including emergency care.
Healthy children 5 to 18 years of age are not at increased risk of complications from influenza. However, children in general tend to have the highest rates of illness in the general population during an influenza outbreak. They also serve as a major source of influenza transmission in the community.
Influenza has a significant impact on school-age children and their environment - missed school, the need to visit medical institutions, and parents missing work days. Also, children often receive additional antibiotics due to the flu.
Excess mortality and morbidity among people at risk
There have been no systematic calculations of excess mortality from influenza in the Russian Federation.
But some studies show that it exists. For example, Saltykova T.S. the incidence of influenza and delayed mortality in people over 60 years of age in the city of Moscow were analyzed (abstract for the degree of candidate of medical sciences, Moscow - 2010). To assess the epidemiological significance of influenza in 1992 - 2007. in Moscow, materials contained in annual official statistical sources were analyzed.
It has been established that the greatest risk of dying from
- patients over 75 years of age have atherosclerotic heart disease after influenza;
- acute myocardial infarction 75-79 and over 85 years of age;
- cerebrovascular diseases - at the age of 70-74, and over 80 years;
- bronchopulmonary pathology - at the ages of 70-74 and 80-84 years,
- pneumonia – aged 65-69 and over 80 years (p < 0.001).
It was found that during the study period (from 1999 to 2005), the average age of the deceased gradually increased (from 77.9 to 78.4 years). A connection has been established between delayed mortality from influenza and atherosclerotic heart disease, acute myocardial infarction, acute cerebrovascular accident - the correlation coefficient is 0.37; 0.31; 0.37 respectively.
Calculation methods have established that annually by vaccinating people over 60 years of age with chronic diseases, it is possible to prevent 246 cases of delayed death from cardiovascular diseases among elderly residents of Moscow.
Sources
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- Esparza J. Lessons From History: What Can We Learn From 300 Years of Pandemic Flu That Could Inform the Response to COVID-19? // Am J Public Health - 2021 - Vol110 - N8 - p.1160-1161; PMID:32639922
- Oancea SC., Watson IW. The association between history of screening for cancer and receipt of an annual flu vaccination: Are there reinforcing effects of prevention seeking? // Am J Infect Control - 2021 - Vol47 - N11 - p.1309-1313; PMID:31253553
- Lycett SJ., Duchatel F., Digard P. A brief history of bird flu. // Philos Trans R Soc Lond B Biol Sci - 2021 - Vol374 - N1775 - p.20180257; PMID:31056053
- Flecknoe D., Charles Wakefield B., Simmons A. Plagues & wars: the 'Spanish Flu' pandemic as a lesson from history. // Med Confl Surviv - 2021 - Vol34 - N2 - p.61-68; PMID:29764189
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