Key facts
Seasonal influenza (the flu) is an acute respiratory infection caused by influenza viruses. It is common in all parts of the world. Most people recover without treatment.
Influenza spreads easily between people when they cough or sneeze. Vaccination is the best way to prevent the disease.
Symptoms of influenza include acute onset of fever, cough, sore throat, body aches and fatigue.
Treatment should aim to relieve symptoms. People with the flu should rest and drink plenty of liquids. Most people will recover on their own within a week. Medical care may be needed in severe cases and for people with risk factors.
There are 4 types of influenza viruses, types A, B, C and D. Influenza A and B viruses circulate and cause seasonal epidemics of disease.
Symptoms of influenza usually begin around 2 days after being infected by someone who has the virus.
Symptoms include:
The cough can be severe and can last 2 weeks or more.
Most people recover from fever and other symptoms within a week without requiring medical attention. However, influenza can cause severe illness or death, especially in people at high risk.
Influenza can worsen symptoms of other chronic diseases. In severe cases influenza can lead to pneumonia and sepsis. People with other medical issues or who have severe symptoms should seek medical care.
Hospitalization and death due to influenza occur mainly among high-risk groups.
In industrialized countries most deaths associated with influenza occur among people aged 65 years or older (1).
The effects of seasonal influenza epidemics in developing countries are not fully known, but research estimates that 99% of deaths in children under 5 years of age with influenza related lower respiratory tract infections are in developing countries (2).
All age groups can be affected but there are groups that are more at risk than others.
Epidemics can result in high levels of worker/school absenteeism and productivity losses. Clinics and hospitals can be overwhelmed during peak illness periods.
Seasonal influenza spreads easily, with rapid transmission in crowded areas including schools and nursing homes. When an infected person coughs or sneezes, droplets containing viruses (infectious droplets) are dispersed into the air and can infect persons in close proximity. The virus can also be spread by hands contaminated with influenza viruses. To prevent transmission, people should cover their mouth and nose with a tissue when coughing and wash their hands regularly.
In temperate climates, seasonal epidemics occur mainly during winter, while in tropical regions, influenza may occur throughout the year, causing outbreaks more irregularly.
The time from infection to illness, known as the incubation period, is about 2 days, but ranges from 1–4 days.
Most cases of human influenza are clinically diagnosed. However, during periods of low influenza activity or outside of epidemics situations, the infection of other respiratory viruses (e.g. SARS-CoV-2, rhinovirus, respiratory syncytial virus, parainfluenza and adenovirus) can also present as influenza-like illness (ILI), which makes the clinical differentiation of influenza from other pathogens difficult.
Collection of appropriate respiratory samples and the application of a laboratory diagnostic test is required to establish 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 is commonly performed using direct antigen detection, virus isolation, or detection of influenza-specific RNA by reverse transcriptase-polymerase chain reaction (RT-PCR). Various guidance on the laboratory techniques is published and updated by WHO.
Rapid diagnostic tests are used in clinical settings, but they have lower sensitivity compared to RT-PCR methods and their reliability depends largely on the conditions under which they are used.
Most people will recover from influenza on their own. People with severe symptoms or other medical conditions should seek medical care.
People with mild symptoms should:
People at high risk or with severe symptoms should be treated with antiviral medications as soon as possible. They include people who are:
The WHO Global Influenza Surveillance and Response System (GISRS) monitors resistance to antivirals among circulating influenza viruses to provide timely evidence for national policies related to antiviral use.
Vaccination is the best way to prevent influenza.
Safe and effective vaccines have been used for more than 60 years. Immunity from vaccination goes away over time so annual vaccination is recommended to protect against influenza.
The vaccine may be less effective in older people, but it will make the illness less severe and reduces the chance of complications and death.
Vaccination is especially important for people at high risk of influenza complications and their carers.
Annual vaccination is recommended for:
Other ways to prevent influenza:
Vaccines are updated routinely with new vaccines developed that contain viruses that match those circulating. Several inactivated influenza vaccines and recombinant influenza vaccines are available in injectable form. Live attenuated influenza vaccines are available as a nasal spray.
WHO, through the Global Influenza Programme and GISRS, in collaboration with other partners, continuously monitors influenza viruses and activity globally, recommends seasonal influenza vaccine compositions twice a year for the northern and southern hemisphere influenza seasons, guides countries in tropical and subtropical areas as to which formulation vaccines to use, supports decisions for timing of vaccination campaigns, and supports Member States to develop prevention and control strategies.
WHO works to strengthen national, regional and global influenza response capacities including diagnostics, antiviral susceptibility monitoring, disease surveillance and outbreak response, to increase vaccine coverage among high-risk groups, and to support research and development of new therapeutics and other countermeasures.
References
1. Estimates of US influenza-associated deaths made using four different methods.
Thompson WW, Weintraub E, Dhankhar P, Cheng OY, Brammer L, Meltzer MI, et al. Influenza Other Respi Viruses. 2009;3:37-49
2. Global burden of respiratory infections due to seasonal influenza in young children: a systematic review and meta-analysis.
Nair H, Abdullah Brooks W, Katz M et al. Lancet 2011; 378: 1917–3
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