Seasonal Influenza (Human)

Primary reference(s)

WHO, 2018. Influenza (seasonal). World Health Organization (WHO). Accessed 8 November 2020.

Additional scientific description

Seasonal influenza is most often caused by type A or B influenza viruses. The constant evolving nature of influenza viruses makes influenza among the top few infectious hazards with significant impact. Epidemics occur mainly during winter, from October to March in the northern hemisphere and April to September in the southern hemisphere. In tropical and subtropical countries, seasonal influenza can happen all year round (WHO, 2020).

Seasonal influenza outbreaks are caused by small changes on the surface of viruses that have already circulated, and to which people have some immunity (WHO, 2018).

Symptoms include sudden onset of fever, cough (usually dry), headache, muscle and joint pain, sore throat and a runny nose. The cough can be severe and can last two or more weeks. Most people recover from fever and other symptoms within a week without requiring medical attention (WHO, 2018).

Seasonal influenza can cause severe illness or death in high-risk groups, such as pregnant women, children younger than five years, people older than 65 years, and people with chronic medical conditions. People with increased risk of exposure to influenza, such as children in day care centres and schools and workers in health care facilities, can transmit infections to others if they become ill (WHO, 2018).

Patients who are not from a high-risk group should be managed with ‘symptomatic treatment’ and are advised, if symptomatic, to stay home to minimise the risk of infecting others in the community. Patients known to be at high risk for developing severe or complicated illness, should be treated with antiviral drugs in addition to symptomatic treatment as soon as possible. Patients with severe or progressive clinical illness associated with suspected or confirmed influenza infection (i.e., clinical syndromes of pneumonia, sepsis or exacerbation of chronic underlying disease) should be treated with antiviral drugs as soon as possible (WHO, 2017, 2018).

Most cases of human influenza are clinically diagnosed. Collection of appropriate respiratory samples and the application of a laboratory diagnostic test is required to establish a definitive diagnosis. Laboratory confirmation of influenza virus from throat, nasal and nasopharyngeal secretions or tracheal aspirate or washings is commonly performed using direct antigen detection, virus isolation, or by reverse transcriptase-polymerase chain reaction (RT-PCR) (WHO, 2018).

The World Health Organization (WHO) recommends annual vaccination with vaccines containing updated formulation for high-risk groups ideally before the season begins. Getting vaccinated at any time during the season can still help prevent flu infections (WHO, 2020).

Metrics and numeric limits

Worldwide, annual epidemics are estimated to result in about 3 to 5 million cases of severe illness, and about 290,000 to 650,000 respiratory deaths (WHO, 2018).

In industrialised countries most deaths associated with influenza occur among people of 65 years of age or older. Epidemics can result in high levels of absenteeism and productivity losses. Clinics and hospitals can be overwhelmed during peak illness periods (WHO, 2018).

The effects of seasonal influenza epidemics in developing countries are not fully known, but research estimates that 99% of deaths in children under five years of age with influenza-associated lower respiratory tract infections are found in developing countries (WHO, 2017).

Examples of drivers, outcomes and risk management

The most effective way to prevent infection with seasonal influenza virus is vaccination. Safe and effective vaccines have been used for more than 60 years. Because the influenza virus continuously mutates, a new vaccine is required each year, and those in risk groups must again be vaccinated (Walsh, 2017).

Globally coordinated surveillance of influenza, timely sharing of influenza virus data and associated information, and innovative research are key to addressing the problem of influenza. Global influenza surveillance has been conducted through the World Health Organization (WHO) Global Influenza Surveillance and Response System (GISRS) since 1952. GISRS is a system fostering global confidence and trust for over half a century, through effective collaboration and sharing of viruses, data and benefits based on Member States’ commitment to a global public health model (WHO, no date).

The aim of GISRS is to protect people from the threat of influenza by functioning: as a global mechanism of surveillance, preparedness and response for seasonal, pandemic and zoonotic influenza; as a global platform for monitoring influenza epidemiology and disease; and as a global alert for novel influenza viruses and other respiratory pathogens (WHO, no date).