Live attenuated vaccines are composed of live, attenuated microorganisms that cause limited or no infection in their host upon administration. The causative agent or microorganism used for the development of live attenuated vaccine is live but it has lost its natural ability to cause the disease it is known for. Microbes used for live attenuated vaccines are sufficient enough to induce an immune response but insufficient to cause any infectious disease in the vaccinated individual. Live attenuated vaccines are made by passing the disease causing virus through a series of cell culture techniques or embryonated chicken egg method (e.g. chicken embryo) under controlled laboratory conditions that make the organism less virulent. Most of the pathogens used for the production of live attenuated vaccines are active viruses that have been cultivated under conditions that disable or inactivate their virulent properties. In some cases, closely related but less dangerous or less-pathogenic microbe can be used to produce immune response in the individual. Live attenuated vaccines are very efficacious; and they induce a protective form of immunity in the individual being vaccinated. Examples of live attenuated vaccines include: oral polio vaccine, yellow fever vaccine, and MMR vaccine (a combination of measles, mumps and rubella vaccine).
MERITS OF LIVE ATTENUATED VACCINES
- Very small doses (usually a single dose) are required to induce immunity.
- Booster doses are not needed for live attenuated vaccines.
- Antibody formation is very fast within the first seven (7) days of administration of live attenuated vaccines; and the antibodies formed can stay for a long time and can last for a lifetime. This is because, as the virus continues to replicate in its host, so will it continue to induce the immune system to produce potent antibodies in advance.
DEMERITS OF LIVE ATTENUATED VACCINES
- A major concern that must be considered in the use of live attenuated vaccine is the possibility of the virus used for the vaccine development to revert to a virulent form capable of causing disease in the vaccinated individual. Mutations can occur when the vaccine virus replicates in the host (especially in immunocompromised individuals – whose immune system have been weakened), and this may result in a more virulent form or strain of the organism that attacks the host.
- Since they are live and their activity depends on their viability, the storage conditions must be strictly adhered to (i.e. maintenance of cold chain of temperatures between 2oC- 8 oC).
- Administration of live vaccines to immunosuppressed individuals may cause serious illness or even death. And thus their usage is usually restricted in immunocompromised individuals.
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