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Flu vaccines

Vaccines against influenza (flu) have been around for 50 years. Despite this, the efficacy of flu vaccines is still questioned, and the ability of vaccines to limit epidemic infection has not been proven.

Types of vaccine

1. Whole virus vaccines - whole inactivated virus vaccines were the first influenza vaccines to be produced. The currently circulating strain of influenza is inoculated into embryonated eggs, harvested 2-3 days later and inactivated. This vaccine confers protection in 60-90% of cases and the protection lasts for 1-5 years, depending on the vaccine strain and the age of the person getting the vaccine. However, the subsequent infecting virus may show slow antigenic drift and the vaccine induced antibody will be less effective in conferring protection against the new strains.
   
   
2. Split virus vaccines - Because of the high incidence of reactions seen in people receiving vaccines given as whole, inactivated virus vaccine, attempts have been made to produce a vaccine which is less reactogenic but conserving immunogenicity. Split vaccines were prepared inactivated particles disrupted with detergents. These vaccines have been shown to induce fewer side effects in the person receiving vaccines and are just are immunogenic as whole virus vaccine. Whole virus vaccine should not be used in children.
   
   
3. Subunit virus vaccines - subunit vaccines have been prepared which contained only the HA and NA antigens. These are used in aqueous suspension or may be absorbed to carriers such as alhydrogel. Volunteers given subunit vaccines experienced fewer reactions than those given whole virus vaccines and those given aqueous vaccine experienced fewer reactions than those given the absorbed subunit vaccine. Therefore, the best vaccines available at present are the aqueous subunit vaccines, although some authorities have questioned the effectiveness of subunit vaccines.
   
   
4. Live attenuated vaccines - there is experimental evidence that immunization with live, attenuated influenza virus vaccines induce a solid immunity than do inactivated vaccines. Normal methods for attenuation, such as repeated passages and temperature adaptation require a long period to complete, and probably too long for the vaccine to become available for immunization against the current influenza strain. To circumvent this problem, already attenuated strains have been mixed with wild-type virus to produce recombinants which contain the RNA fragments which code for wild-type HA and NA, and all the other genetic material form the attenuated strain. These recombinants can be produced relatively quickly. When given intranasally it produces few side effects.