Seasonal flu
Seasonal influenza is an acute viral infection caused by an influenza virus.
There are 3 types of seasonal flu - A, B and C. Influenza A viruses are subdivided into subtypes based on the different kinds and combinations of virus surface proteins. Of the many subtypes of influenza A viruses, subtypes A(H1N1) and A(H3N2) are currently circulating in humans.
The circulating influenza A(H1N1) virus is also written A(H1N1)pdm09 since it caused the 2009 pandemic and then replaced the seasonal influenza A(H1N1) virus circulating before 2009. Only influenza A viruses have caused pandemics.
The circulating influenza B viruses can be divided into 2 main groups, or lines, called the B/Yamagata and B/Victoria lines. Influenza B viruses are not classified into subtypes.
Influenza C virus is very rarely detected and usually causes only mild infections, so its public health impact is less significant.
Signs and symptoms
Seasonal influenza is characterized by a sudden onset of high fever, cough (usually dry), headache, muscle and joint pain, general malaise, sore throat and runny nose. The cough can be severe and last up to 2 weeks or more.
Most people recover within a week without the need for medical treatment. But the flu can cause serious illness or death in high-risk people (see below). The incubation period (the time between illness and the onset of symptoms) is about two days.
Populations at risk
Each year, influenza epidemics can severely affect all age groups, but the highest risk of complications is for pregnant women, children 6-59 months of age, the elderly, and individuals of any age with certain chronic conditions such as HIV/AIDS, asthma, or chronic heart or lung disease, as well as health care workers.
Transmission
Seasonal influenza spreads easily, and transmission in facilities that many people attend, such as schools or nursing homes, can be rapid. When an infected person coughs or sneezes, droplets carrying the virus (and infection) are released into the air and reach and breathe in people in close proximity.
The virus can also be spread through the hands. To prevent transmission, cover your mouth and nose with a tissue when you cough and wash your hands regularly.
Seasonal epidemic and disease burden
In temperate climates, seasonal epidemics occur mainly during the winter, while in tropical regions, influenza can occur throughout the year, with more irregular outbreaks.
The disease can range from begenin to severe and can even lead to death. Hospitalizations and deaths occur mainly in high-risk groups. Globally, these annual epidemics are responsible for approximately 5 million cases of severe disease and 290,000 to 650,000 deaths.
Most influenza-related deaths in industrialized countries occur among people aged 65 years or older.1 Epidemics can result in high levels of work absenteeism and lost productivity. Clinics and hospitals may be overwhelmed during the peak of the disease.
The precise effects of seasonal influenza epidemics in developing countries are not known, but research estimates indicate that 99% of deaths of children under 5 years of age with influenza-associated lower respiratory tract infections occur in developing countries.2
Prevention
The most effective way to protect against disease or serious outcomes is vaccination. Safe and effective vaccines have been available and in use for more than 60 years. In healthy adults, the influenza vaccine can provide adequate protection. In older people, on the other hand, influenza vaccine may be less effective in preventing illness, but can reduce the severity of the disease and the incidence of complications and death.
Vaccination is especially important for people at higher risk of serious complications from influenza, and for people who live with or care for people at high risk.
WHO recommends annual vaccination for (in order of priority):
Pregnant women at any stage of pregnancy;
children 6 months to 5 years of age;
the elderly (≥ 65 years old);
people with chronic conditions;
health workers.
Influenza vaccination is most effective when the vaccine viruses match the circulating viruses. Because influenza viruses are constantly changing, the WHO Global Influenza Surveillance and Response System (GISRS) - a network of national influenza centers and WHO collaborating centers around the world - monitors influenza viruses circulating in humans and updates the composition of influenza vaccines twice a year.
For many years, WHO has updated twice a year its recommendations on the composition of trivalent vaccines that target the three most representative circulating virus types (two A virus subtypes and one B virus type).
Since the 2013-2014 influenza season in the Northern Hemisphere, WHO recommendations also include a fourth component for quadrivalent vaccines. Quadrivalent vaccines contain a second influenza B virus in addition to the viruses contained in conventional trivalent vaccines and are expected to provide increased protection against influenza B virus infections. A number of inactivated influenza vaccines and recombinant influenza vaccines are available in injectable form. Live attenuated influenza vaccine is administered intranasally.
Treatment
Antivirals for influenza are available in some countries and can prevent severe complications and deaths. Ideally, they should be administered at the onset of illness (within 48 hours of the onset of symptoms). There are 2 classes of antivirals:
Influenza neuraminidase inhibitors (oseltamivir and zanamivir, but also peramivir and laninamivir, which are approved in several countries).
Adamantanes, M2 protein (amantadine and remantadine) inhibitors, but virus resistance to these drugs, limiting the effectiveness of treatment, has been frequently reported.
WHO is monitoring the antiviral susceptibility of circulating influenza viruses with a view to providing timely guidance on the use of antivirals in clinical management and possibly chemoprophylaxis.
Currently, the majority of circulating influenza viruses are resistant to adamantanes, and the WHO recommends neuraminidase inhibitors as first-line therapy for people requiring antiviral therapy.
WHO action
Through the WHO Global Influenza Surveillance and Response System and in collaboration with its partners, WHO monitors the global influenza situation, recommends twice a year the composition of seasonal influenza vaccine for the northern and southern hemispheres, and supports Member States' efforts to develop prevention and control strategies.
WHO works to strengthen national and regional capacity for influenza diagnosis, antiviral susceptibility surveillance, disease surveillance, outbreak response, and increasing immunization coverage in high-risk groups.
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