Live virus vaccines are prepared from attenuated strains that are completely or almost devoid of pathogen city but are capable of inducing a protective immune response.

Since a related virus from different host cannot cause disease in the new host it can also be used in live virus vaccines. Use of cowpox virus in the prevention of smallpox disease is one of the best examples for this.

A pathogen can multiply in unnatural host (human) without any pathogenesis, but provide continuous antigenic stimulation over a period of time. Failure with these vaccines is rare but inadequate storage or administration may cause failure.

Another possibility for the failure is interference of related viruses, as is suspected in the case of oral polio vaccine in developing countries.


Attenuation is a technique used for reducing the virulence of a virus, while using it in vaccine preparation. It is the counterpart of the vaccines produced by ‘killing’ the virus (inactivated vaccine). Several methods have been used to attenuate viruses for vaccine production.

They are as follows:

a. Passage of the virus in “unnatural host” or host cell –

Major vaccines for man and animals are developed in this way. After repeated passages in unnatural host, the virus loses its virulence; this virus can be administered to the natural host for immunization purpose.


For example vaccine for yellow fever is developed by passing 17D strain of yellow fever virus in mice and then in chick embryos before using it for vaccination.

In the same way Polio viruses are passed in monkey kidney cells and measles viruses are passed in chick embryo fibroblasts. Human diploid cells [aborted babies] such as the WI-38 and MRC-5 are now widely used for the same purpose.

b. Development of temperature sensitive mutants –

This method may be used in combination with the above method for attenuation of pathogen. Variation in the temperature of the host helps in attenuation of the pathogen.


c. Use of an unnatural route –

The virulence of the virus is often reduced when administered through an unnatural route. Hence administration of pathogenic or partially attenuated virus through an unnatural route also helps in immunization without any pathological activity in host. This principle is used in the immunization of military recruits against adult “Respiratory Distress Syndrome” using enterically coated live adenovirus.

Potential Safety Problems with Live Vaccines

1)Under attenuation;


2) Mutations leading to reversion of virulence;

3) Preparation instability;

4) Contamination of viruses in cultured cells;

5) Heat liability;


6) Not safe for pregnant women and immunocompromized patients such as AIDs patients, patents using immune suppressive drugs after transplantation etc.