Types of seasonal influenza vaccine
Two types of influenza vaccine are widely available: inactivated influenza vaccines (IIV) and live attenuated influenza vaccines (LAIV). Traditionally, influenza vaccines (both IIV and LAIV) have been produced to protect against 3 different seasonal influenza viruses (also called trivalent vaccines). In most countries this is still the case and the current trivalent vaccines contain influenza A(H3N2), pandemic A(H1N1) and 1 of 2 influenza B lineage viruses. However, recently vaccines which protect against 4 different viruses, including both influenza B lineage viruses (quadrivalent vaccines), have become available in some countries.
Regardless of the type or composition of seasonal influenza vaccine, vaccination should be administered annually to provide optimal protection against infection.
Annual selection of viruses
The composition of influenza vaccines (both IIV and LAIV) are updated annually by WHO based on information gathered from the Global Influenza Surveillance and Response System (GISRS), a partnership of 141 national influenza centres in 111 countries, 6 WHO collaborating centres and 4 WHO essential regulatory laboratories.
The WHO GISRS collects and analyses influenza virus samples from around the world on an ongoing basis. Each year, 1 or more components of the vaccine designated for the coming influenza season in the northern and/or southern hemisphere might be changed to reflect the most frequent and recent circulating influenza A and B viruses.
Since vaccine production takes about 6 months, each year the influenza vaccine is produced under great time pressure requiring timely submission of viruses to the WHO GISRS. The production of seasonal influenza vaccines is based on viral propagation in embryonated eggs or cell cultures.
Inactivated influenza vaccines (IIV)
IIV is approved for use in persons 6 months and older, including pregnant women and persons with chronic medical conditions. One dose, injected into the deltoid thigh or muscle, is recommended. However, children aged 6 months to 8 years who have not received seasonal influenza vaccine during the previous influenza season should receive 2 doses administered at least 4 weeks apart. Influenza vaccination in pregnancy will protect both the mother and her newborn against influenza.
The viruses included in these vaccines are inactivated so they do not cause influenza, although minor side effects may occur including local reactions at the injection site. Persons without previous exposure to influenza vaccine antigens may experience transient fever, malaise, myalgia (muscle pain) and other systemic adverse events.
Live attenuated influenza vaccines (LAIV)
LAIV is approved for use only in persons aged 2–49 years who do not have underlying medical conditions. The vaccine should, however, not be administered to pregnant women. LAIV is given as a nasal spray, 1 dose only; but children aged 2–8 years who have not received seasonal influenza vaccine during the previous influenza season should receive 2 doses, at least 4 weeks apart.
LAIV is made from attenuated, or weakened, viruses and does not cause influenza, although it can cause mild signs or symptoms (including rhinorrhoea, nasal congestion, fever or sore throat). Most common side effects from the vaccine are mild and transient compared to symptoms of influenza infection.