There are in principle two types of seasonal influenza vaccines. The first is the traditional flu shot, which is an inactivated vaccine that does not contain live virus. It is given by injection, usually in the upper arm muscle. Inactivated vaccines are available in three forms: whole-virus, split-virus, and subunit-virus vaccines. Whole-virus vaccines are classically produced by injecting influenza virus into chicken eggs and harvesting the fluids surrounding the chick embryo 2-3 days later. The virus is subsequently purified and inactivated or ‘killed’. Split-virus vaccines are essentially made in the same way, but the virus is treated with a detergent and therefore these preparations contain essentially all viral structural proteins and disrupted portions of the viral membrane. Subunit vaccines are also essentially made in the same way, but at the end of the procedure the surface proteins (H and to a lesser extent N) are purified from the virus preparation. Split-virus and subunit vaccines are the most widely used influenza vaccines since they combine a high level of effectiveness with little or no side effects.
A newer vaccine is administered via a nasal spray. It is directed against the same strains of virus as the classical seasonal influenza vaccine. It is also produced in chicken eggs, but differs in that it contains attenuated or weakened live influenza viruses instead of killed viruses. The vaccine is therefore referred to as Live-Attenuated Influenza Vaccine (LAIV). Attenuated vaccine viruses do not cause influenza, but like the inactivated vaccines, do stimulate the immune system to produce a specific immune response and immune memory. The LAIV was first made available in the US. In 2011, a live attenuated influenza vaccine for intranasal use was also approved in the EU for children and adolescents (2-17 years of age). From this season onwards, all LAIV vaccines available are quadrivalent combination vaccines containing two influenza A strains (H1N1 and H3N2) and two influenza B strains.