INFLUENZA
Influenza is an acute respiratory disease caused by infection of the respiratory tract by seasonal influenza viruses. Of the four types of influenza viruses (A-D), three types (A, B, and C) infect and cause illness in humans. Types A and B, which cause global epidemics, are referred to as seasonal influenza viruses. More than 32 million cases and 5.7 million hospitalizations occur worldwide each year, with the highest incidence in people aged 65 and older.
Influenza A virus is classified into subtypes based on the proteins that project through the viral particle’s membrane: hemagglutinin (H) and neuraminidase (N). Eighteen hemagglutinin subtypes and eleven neuraminidase subtypes have been identified. Currently, influenza A (H1N1) and A (H3N2) viruses circulate among the global population. Epidemics caused by the A (H3N2) subtype are associated with high mortality in older adults and children. The economic cost associated with seasonal influenza in the U.S. is $11.2 billion, directly related to medical care.
Seasonal influenza A and B viruses evolve to evade immunity through modifications in the amino acids of their membrane proteins, allowing them to escape antibodies induced by prior infections, vaccinations, or both. This evolutionary process is known as antigenic variation and drives annual influenza epidemics, highlighting the importance and necessity of yearly vaccination.
The average incubation period is 1.4 days for influenza A and 0.6 days for influenza B. Symptoms peak 2 to 3 days after inoculation. In general, seasonal influenza A and B viruses are transmitted over short distances (1 to 2 meters) from person to person through large droplets (greater than 5 microns) and small particle droplet nuclei (less than 5 microns, aerosols), which are expelled by infected individuals when coughing. The concentration of viral particles in the upper respiratory tract is highest on days 1 and 2 after symptom onset and gradually decreases over the next 3 days. Young children can shed the virus for up to a week, and severely immunocompromised patients may do so for weeks or even months.
Clinical diagnosis is often inadequate due to the similarity of symptoms caused by other circulating pathogens. Therefore, it is important to request diagnostic tests and avoid the unnecessary use of antibiotics due to the risk of superinfection and complications associated with their inappropriate use. Additionally, the use of amantadine-based antivirals should be avoided due to the high level of resistance of the influenza A virus, and corticosteroids should not be used due to the increased mortality associated with their administration.
The rapid antigen diagnostic test (20 minutes) should ideally be performed within the first 4 days of symptoms. It has low sensitivity (40%) and high specificity, with a low negative predictive value. Therefore, the risk of false negatives is high. In cases of high clinical suspicion during seasonal peaks, if the rapid test result is negative, a highly sensitive (95%) and specific (99%) real-time polymerase chain reaction (RT-PCR) molecular test should be performed.
The clinical benefit of treatment is greatest when initiated immediately after symptom onset, as it reduces viral proliferation in the respiratory tract. If started within the first 2 days, it shortens the duration of illness by 18 to 30 hours, reduces the risk of respiratory complications by 44%, and lowers the risk of hospitalization by 63%.
DR. JAVIER BENJAMÍN ZATARAIN GUERRERO
Internal Medicine Physician with a Subspecialty in Critical Care and Intensive Therapy, Certified by the Internal Medicine and Critical Care Boards
Available 24 hours a day, 7 days a week, 365 days a year.
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