Influenza: epidemiology

The main source of the influenza virus is an influenza-infected person with a clinically expressed or erased form of the disease course. The epidemiological significance of a sick person is determined by the amount of the virus in the upper respiratory tract and the severity of the catarrhal syndrome. In the incubation period of the disease, the dispersion of viruses is not intensive. The absence of catarrhal symptoms limits the spread of viruses in the environment (therefore the epidemiological danger of the patient is negligible). Sick children with severe illness are the most intense source of the virus. However, they represent less epidemiological danger than adults with an easier disease course, since adults can have multiple contacts at home, in transport and at work. After 7 days of the disease, the virus can not be isolated from the patient in most cases.

Long-term viral isolation is detected in patients with severe and complicated course of the disease. Under certain conditions, the viruses of serotypes H1N1, H2N2 and H3N2 can be isolated from patients within 3-4 weeks, and influenza B viruses up to 30 days. Residual catarrhal phenomena in the airways contribute to the transmission of the pathogen to others, so convalescent can serve as a source of the virus in healthy groups. The source of the virus can also be people with a clinically not expressed form of the infectious process and transient virus carriers.

The epidemiological significance of an infected person directly depends on the degree of severity of catarrhal symptoms. According to laboratory data, 50-80% of adults infected with influenza viruses (as evidenced by the increase in the titer of specific antibodies) do not seek medical help (in children this percentage is less). Many people infected with the influenza virus do not notice signs of the disease or carry it in an easy form. This group of patients represents the epidemiologically most significant source of the pathogen.

The mechanism of transmission of the influenza virus is aerosol. The transmission path is airborne. The role of the air-dust path is small. From the damaged cells of the epithelium of the respiratory tract, the virus enters the air with droplets of saliva, mucus and sputum during breathing, talking, crying, coughing and sneezing. The safety of the influenza virus in the air depends on the degree of dispersion of the aerosol containing the virus particles, as well as on the effects of light, moisture, and high temperatures on it. It is possible to infect through objects contaminated with the patient's discharge (toys, dishes, towels, etc.).

The influenza virus retains its vitality and virulence in the living quarters for 2-9 hours. With a decrease in the relative humidity of the air, the survival time of the virus increases, and with an increase in air temperature to 32 ° C it decreases to 1 hour. Data on the survival of the virus influenza in the environment. Influenza A (Brazil) 11/78 (H1N1) and B (Illinois) 1/79 viruses persisted on metal and plastic for 24-48 hours, and on paper, carton and tissues for 8-12 hours. The viruses remained viable and virulent in the hands person within 5 minutes. In sputum, the influenza virus retains virulence 2-3 weeks, and on the surface of the glass - up to 10 days.

Thus, the risk of infection of people with the influenza virus by airborne droplets remains on average within 24 hours after its isolation from the patient's body.

People's susceptibility to flu is high. To date, there is no convincing evidence of the presence of genetically determined resistance of humans to influenza A and B viruses. However, this is true for the first contact with the pathogen. Due to the wide spread of the influenza virus in newborns, antibodies specific for the influenza virus, derived from the mother through the placenta and milk, determine the temporary resistance. Titres of antiviral antibodies in the blood of the child and mother are almost identical. Maternal antibodies to influenza virus are detected in children receiving breast milk before 9-10 months of life (however, their titer gradually decreases), and with artificial feeding - only up to 2-3 months. Passive immunity received from the mother is inadequate, so when outbreaks occur in maternity hospitals, the incidence of newborns is higher than among their mothers. Postinfectious immunity is type-specific: for influenza A it persists for at least three years, with influenza B - 3-6 years.

Influenza is characterized by epidemic, and often pandemic spread in a relatively short time, which depends on the following factors:

• high incidence of mild forms of the disease and a short incubation period;
• aerosol mechanism of transmission of the pathogen;
• high susceptibility of people to the pathogen;

On the appearance in each epidemic (pandemic) of a new serovar pathogen, to which the population does not have immunity; on the type-specificity of post-infection immunity, which does not give protection from other strains of the virus. Antigenic drift causes the frequency of epidemics (duration 6-8 weeks). Epidemic upsurge in the autumn-winter period is associated with the general factors determining the seasonal unevenness of the incidence of ARI. The result of antigenic schift is the emergence of pandemics.

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