Influenza - Epidemiology

The main source of the influenza virus is a person with influenza with clinically expressed or latent forms of the disease. The epidemiological significance of a sick person is determined by the amount of virus in the discharge of the upper respiratory tract and the severity of the catarrhal syndrome. During the incubation period of the disease, the dissemination 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 insignificant). Sick children with a severe course of the disease are the most intensive source of the virus. However, they pose a lower epidemiological danger than adults with a milder course of the disease, since adults can have multiple contacts at home, in transport and at work. After 7 days of the disease, it is not possible to isolate the virus from the patient in most cases.

Long-term excretion of the virus is detected in patients with severe and complicated course of the disease. Under certain conditions, viruses of serotypes H1N1, H2N2 and H3N2 can be excreted from patients for 3-4 weeks, and influenza B viruses - up to 30 days. Residual catarrhal phenomena in the respiratory tract contribute to the transmission of the pathogen to others, so convalescents can serve as a source of the virus in healthy groups. Persons with a clinically unexpressed form of the infectious process and transient virus carriers can also be a source of the virus.

The epidemiological significance of an infected person directly depends on the severity of catarrhal symptoms. According to laboratory studies, 50-80% of adults infected with influenza viruses (as evidenced by the growth of the titer of specific antibodies) do not seek medical care (in children, this percentage is lower). Many people infected with the influenza virus do not notice signs of the disease or suffer from it in a mild form. This group of patients is the epidemiologically most significant source of the pathogen.

The mechanism of transmission of the flu virus is aerosol. The transmission route is airborne. The role of the airborne dust route is insignificant. From damaged cells of the epithelium of the respiratory tract, the virus enters the air with droplets of saliva, mucus and sputum when breathing, talking, crying, coughing and sneezing. The safety of the flu virus in the air depends on the degree of dispersion of the aerosol containing virus particles, as well as on its exposure to light, moisture, high temperatures. Infection is possible through objects contaminated with the patient's secretions (toys, dishes, towels, etc.).

The influenza virus retains its viability and virulence in residential premises for 2-9 hours. With a decrease in relative air humidity, 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 time of the influenza virus on environmental objects are of great importance. Influenza viruses A (Brazil) 11/78 (H1N1) and B (Illinois) 1/79 survived on metal and plastic for 24-48 hours, and on paper, cardboard and fabrics - 8-12 hours. The viruses remained viable and virulent on human hands for 5 minutes. In sputum, the influenza virus retains virulence for 2-3 weeks, and on a glass surface - up to 10 days.

Thus, the risk of people becoming infected with the influenza virus through airborne droplets remains on average for 24 hours after it is released from the patient’s body.

People are highly susceptible to influenza. There is currently no convincing evidence of genetic resistance to influenza A and B viruses in humans. However, this is true for the first contact with the pathogen. Due to the widespread prevalence of the influenza virus, newborns are found to have influenza-specific antibodies received from the mother through the placenta and milk, which provide temporary resistance. The titers of antiviral antibodies in the blood of the child and mother are almost identical. Maternal antibodies to the influenza virus are found in children who receive breast milk for up to 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 incomplete, therefore, during outbreaks of this disease in maternity hospitals, the incidence rate among newborns is higher than among their mothers. Post-infectious immunity is type-specific: with influenza A it lasts for at least three years, with influenza B - 3-6 years.

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

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

About the emergence of a new serovar of the pathogen in each epidemic (pandemic), to which the population has no immunity; about the type-specificity of post-infection immunity, which does not provide protection from other strains of the virus. Antigenic drift determines the periodicity of epidemics (duration 6-8 weeks). Epidemic rises in the autumn-winter period are associated with general factors that determine the seasonal unevenness of the incidence of acute respiratory infections. The result of antigenic shift is the emergence of pandemics.

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