Seasonal flu is an acute respiratory infection caused by influenza viruses circulating around the world. It causes respiratory tract disease. There are 4 types of seasonal influenza viruses, types A, B, C, and D. Influenza A and B viruses circulate and cause seasonal disease outbreaks.

Influenza A viruses are further subdivided according to their combination of hemagglutinin (HA) and neuraminidase (NA), proteins on the surface of the virus. The subtypes A(H1N1) and A(H3N2) currently circulating in humans are influenza viruses. A(H1N1) is also spelled A(H1N1)pdm09 as it caused the pandemic in 2009 and then replaced the seasonal influenza A(H1N1) virus that was circulating before 2009. Only influenza type A viruses are known to cause pandemics. Influenza B viruses are not classified by subtypes but can be divided into lineages. Currently circulating influenza type B viruses belong to the B/Yamagata or B/Victoria lineage. Influenza C virus is less detected and usually causes mild infections, so it is not of public health importance. Influenza D viruses primarily affect cattle and are not known to infect humans or cause disease.


All age groups are affected. Persons at higher risk of serious illness or complications are; pregnant women, children younger than 5 years, elder people, individuals with chronic medical conditions (for example, chronic heart, lung, kidney, metabolic, neurodevelopmental, liver, or hematological diseases), and whose immune system are compromised with suppressing conditions (such as HIV/AIDS, receiving chemotherapy or steroids, or malignancy). In temperate climates, seasonal outbreaks occur predominantly during the winter months, while in tropical regions, influenza can occur throughout the year, causing more irregular outbreaks. Worldwide, these annual outbreaks are estimated to result in approximately 3 to 5 million cases of serious illness and approximately 290,000 to 650,000 respiratory deaths.

Signs And Symptoms

Seasonal flu is characterized by sudden onset of fever, cough (usually dry), headache, muscle and joint pain, severe malaise (feeling unwell), sore throat, and runny nose. The cough can be severe and last 2 or more weeks. The time from infection to illness, known as the incubation period, is about 2 days but ranges from one to four days. Most people recover from fever and other symptoms within a week without medical attention. But the flu can cause serious illness or death, especially in people at high risk. Most influenza-related deaths in industrialized countries occur in people aged 65 and over. Outbreaks can cause high levels of worker/school absenteeism and lost productivity. Clinics and hospitals can be overwhelmed during peak illness periods.

In terms of transmission, seasonal flu spreads easily with rapid transmission in crowded areas such as schools and nursing homes. When an infected person coughs or sneezes, the virus-containing droplets (contagious droplets) spread into the air and can spread up to one meter and infect people in the immediate vicinity who inhale these droplets. To prevent contamination, people should cover their mouth and nose with a tissue when coughing and wash their hands regularly.


Most cases of human flu are diagnosed clinically. However, during periods of low flu activity, infection of other respiratory viruses (e.g., Rhinovirus, respiratory syncytial virus, parainfluenza, and adenovirus) may also present as Influenza-Like Illness (ILI), making it difficult to clinically distinguish influenza from other pathogens.

Appropriate respiratory samples should be taken, and laboratory diagnostic tests should be applied to make a definitive diagnosis. Proper collection, storage, and transport of respiratory specimens is the essential first step for laboratory detection of influenza virus infections. Laboratory confirmation of influenza virus from throat, nasal, and nasopharyngeal secretions or tracheal aspirate or washes is usually accomplished using direct antigen detection, virus isolation, or RT-qPCR. Rapid influenza diagnostic tests (RIDTs) are also used in clinical settings, but they have lower sensitivity compared to RT-qPCR methods and their reliability is highly dependent on the conditions in which they are used.


Besides vaccination and antiviral therapy, public health management includes personal protective measures such as:

  • Regular hand washing with proper drying of hands
  • Good respiratory hygiene – covering mouth and nose when coughing or sneezing, using tissues, and discarding them correctly
  • Early self-isolation of those who feel unwell, have a fever, or have other flu symptoms
  • Avoiding close contact with sick people
  • Avoiding touching the person’s eyes, nose, or mouth

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