These vaccines were genetically prone to instability, resulting in variable degrees of attenuation and cases of influenza infection in some vaccinees. For this reason, this approach was abandoned in favour of inactivated check details whole formulations. Also
first developed during World War II, killed whole-virus vaccines were immunogenic, but remained quite reactogenic, especially in children, where high rates of fever were recorded. This prompted the search for subvirion vaccines. Although whole-cell vaccines are still in use today in some countries, the majority of influenza vaccines manufactured over the last 30–40 years have been based on subunit and split-virus formulations, developed to minimise reactogenicity. These antigens consist of influenza fragments of varying degrees of purity. Some vaccines of this type are purified sub-virus particles (split
vaccines), whereas others are based on highly selected and purified virus proteins or proteins produced from recombinant systems (subunit Selleckchem Ixazomib vaccines). The tolerability profile of these purified antigens is better than that with whole-pathogen vaccines, and their immunogenicity has been satisfactory. One dose of the vaccine is enough for the adult population, probably due to previous exposure to influenza, while two doses of split/subunit vaccines are needed in young children since most of them are naïve to influenza infections. An ongoing challenge with seasonal influenza vaccines that continues to drive vaccine research is limited immunogenicity in the elderly. This is due to the natural process of immunological senescence – a declining ability of the immune system to mount effective immune responses with increasing age. One of the approaches to solving this problem is the use of adjuvants and two seasonal
influenza vaccines, one adjuvanted with an oil-in-water emulsion and the other with a virosome (based on liposome), which became available in Europe Casein kinase 1 in the 1990s. The adjuvanted vaccine improves immune responses in the elderly compared with the traditional non-adjuvanted vaccine. Also in the 1990s, research on live, attenuated influenza vaccines experienced a resurgence as techniques, such as targeted gene deletions and reassortment of related strains, made it possible to produce vaccine strains with specific characteristics. These included cold-attenuated strains that were unable to replicate in the warm (core body temperature) environment of the lungs. This approach permitted the development of a trivalent cold adapted influenza vaccine first licensed in the USA in 2003 and currently approved for healthy children older than 2 years and adults less than 50 years of age. This vaccine, which is delivered intranasally, is updated with new reassortant strains each year to protect against seasonal influenza and is capable of inducing strong immune responses in children.