Seasonal influenza is an acute respiratory infection caused by influenza viruses circulating worldwide. Seasonal epidemics of this disease mainly occur in winter, although in tropical regions, the prevalence of influenza is irregular and may occur throughout the year.

There are four types of seasonal influenza viruses: A, B, C, and D. Influenza A and B viruses cause seasonal epidemics, leading to the occurrence and spread of infectious diseases.

Influenza A viruses are classified into subtypes based on a combination of surface proteins, hemagglutinin (HA), and neuraminidase (NA). When a new and different subtype of influenza A virus emerges with the ability to cause disease and spread effectively among people, a pandemic is triggered. Currently, subtypes of influenza A virus (H1N1) and A(H3N2), and lineage B/Yamagata or B/Victoria of influenza B are circulating globally and causing disease in humans. Influenza C viruses usually cause mild infections, therefore, they are not of public health concern, and influenza D viruses mainly affect cattle and do not typically infect or cause disease in humans.

In terms of transmission, seasonal influenza easily spreads and can be transmitted in crowded areas such as schools and nursing homes. When an infected person coughs or sneezes, virus-containing droplets (infectious droplets) are dispersed in the air and can spread up to one meter, infecting individuals who breathe in close proximity to these droplets. The virus can also be transmitted through contaminated hands. To prevent the transmission of influenza viruses, individuals should cover their mouth and nose with a tissue when coughing and regularly wash their hands.

The incubation period of this disease is approximately 2 days but can vary from 1 to 4 days. Most affected individuals recover from fever and other symptoms within a week without the need for medical care. However, influenza can lead to severe illness or death, especially among high-risk groups such as children under 5 years old, the elderly, pregnant women, individuals with chronic diseases (such as heart, lung, kidney, liver, or hematologic diseases, and individuals with immune system suppression conditions such as AIDS, receiving chemotherapy or steroids, or malignancy). Healthcare workers are also at increased risk of influenza virus infection due to increased exposure to patients and can contribute to further spread of the disease, especially to vulnerable individuals.

The symptoms of seasonal influenza are characterized by a sudden onset of fever, cough (usually dry), headache, muscle and joint pain, severe fatigue (malaise), sore throat, and runny nose. Cough can be severe and last for two weeks or more. For patients not in high-risk groups, treatment focuses on symptom reduction such as fever control. Patients identified as being at high risk of developing severe or complicated illness should seek medical care and be promptly treated with antiviral medications. Globally, it is estimated that annual epidemics result in approximately 3 to 5 million cases of severe illness and around 29,000 to 65,000 respiratory deaths. Hospitalization and death mainly occur among high-risk groups.

To combat this disease and reduce mortality in recent years, seasonal vaccines have been used. Vaccines are biological products that induce active acquired immunity against a specific microbe or virus.

Seasonal influenza

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