Influenza is an acute respiratory infection caused by influenza viruses. According to statistics, between 7.6~1.52 billion people worldwide are infected with influenza every year. Among them, the incidence in adults ranges from 5%~10%, while the incidence of children is much higher, reaching up to 20%~30%. The global influenza strategy released by the World Health Organization in 2019 estimates that out of 1 billion people infected with influenza each year, between 5 to 300 million are severe, and the number of deaths ranges from 29,000 to 650,000, earning it the label of a global health threat.
Influenza is an RNA virus that can be classified into four categories: A, B, C, and D based on its nucleoprotein and matrix protein. Among these categories, the influenza A virus is capable of infecting humans of all age groups, and often causes worldwide pandemics. The primary influenza A viruses responsible for pandemics such as H1N1, H2N2, and H3N2. Compared to influenza B, C, and D viruses, influenza A viruses are known to mutate easily, producing many subtypes. Human avian influenza is an acute respiratory infectious disease caused by specific subtypes of the avian influenza A virus. Gene mutation in the virus can enable it to infect humans, causing symptoms such as high fever, cough, and runny nose, often accompanied by severe pneumonia and organ failure, leading to a very high case of fatality rate. The disease can be transmitted through various routes, including the digestive tract, respiratory tract, skin damage, etc.
Influenza B virus is capable of circulating within the human body, causing seasonal epidemics that mainly affect children and can cause local outbreaks. Recent data indicates that seals can also be infected by the influenza B virus. Influenza C virus, on the other hand, is relatively antigenically stable and can infect both humans and pigs, though the symptoms following infection are typically mild. Influenza D virus mainly affects cattle, and it remains unclear whether it causes morbidity in humans.
Influenza viruses are highly susceptible to mutations due to climate and environmental factors. In addition to local mutation, nucleic acid segmentation can also lead to genetic recombination, resulting in the formation of new virus subtypes through antigenic mutation.
Influenza viruses mutate quickly and are difficult to prevent with vaccines. Therefore, they are currently primarily treated with neuraminidase inhibitors. Neuraminidase is an enzyme composed of proteins on the surface of influenza virus particles and is the most critical enzyme for viral replication and spread. Viral neuraminidase inhibitors are a new class of influenza prevention and treatment drugs with a unique mechanism of action after amantadine and influenza vaccine. They can selectively inhibit the activity of neuraminidase on the surface of respiratory viruses, prevent the replication and release of daughter virus particles in human cells, effectively prevent colds, relieve symptoms, and significantly shorten the duration of influenza when applied within 48 hours in the early days of colds. At present, representative drugs are oseltamivir and zanamivir. However, the clinical preparation of zanamivir is a powder inhaler and can not be taken orally, limiting its use. Therefore, oseltamivir is generally used for treatment in clinical practice.
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