Portal:Viruses/Selected intervention/9

Influenza vaccine strains are usually cultivated in fertilised chicken eggs.

Influenza vaccines include live attenuated and inactivated forms. Inactivated vaccines contain three or four different viral strains selected by the World Health Organization to cover influenza A H1N1 and H3N2, as well as influenza B, and are usually administered by intramuscular injection. The live attenuated influenza vaccine contains a cold-adapted strain and is given as a nasal spray. Most influenza vaccine strains are cultivated in fertilised chicken eggs (pictured), a technique developed in the 1950s; others are grown in cell cultures, and some vaccines contain recombinant proteins. Annual vaccination is recommended for high-risk groups and, in some countries, for all those over six months. As the influenza virus changes rapidly by antigenic drift, new versions of the vaccine are developed twice a year, which differ in effectiveness depending on how well they match the circulating strains. Despite considerable research effort for decades, no effective universal influenza vaccine has been identified. A 2018 meta-analysis found that vaccination in healthy adults decreased confirmed cases of influenza from about 2.4% to 1.1%. However, the effectiveness is uncertain in those over 65 years old, one of the groups at highest risk of serious complications.