1. Live Vaccines

Live virus vaccines are prepared from attenuated strains that are almost or completely devoid of pathogenicity but are capable of inducing a protective immune response. They multiply in the human host and provide continuous antigenic stimulation over a period of time, Primary vaccine failures are uncommon and are usually the result of inadequate storage or administration. Another possibility is interference by related viruses as is suspected in the case of oral polio vaccine in developing countries. Several methods have been used to attenuate viruses for vaccine production.

Use of a related virus from another animal - the earliest example was the use of cowpox to prevent smallpox. The origin of the vaccinia viruses used for production is uncertain.

Administration of pathogenic or partially attenuated virus by an unnatural route - the virulence of the virus is often reduced when administered by an unnatural route. This principle is used in the immunization of military recruits against adult respiratory distress syndrome using enterically coated live adenovirus type 4, 7 and (21).

Passage of the virus in an "unnatural host" or host cell - the major vaccines used in man and animals have all been derived this way. After repeated passages, the virus is administered to the natural host. The initial passages are made in healthy animals or in primary cell cultures. There are several examples of this approach: the 17D strain of yellow fever was developed by passage in mice and then in chick embryos. Polioviruses were passaged in monkey kidney cells and measles in chick embryo fibroblasts. Human diploid cells are now widely used such as the WI-38 and MRC-5. The molecular basis for host range mutation is now beginning to be understood.

Development of temperature sensitive mutants - this method may be used in conjunction with the above method.