Meaning and Principle
Live-attenuated vaccines contain living microorganisms that have been weakened so that they can reproduce inside the body without normally causing disease in a healthy person.
The microorganism may be a virus or bacterium. It is modified through laboratory processes so that its disease-causing ability is greatly reduced while its capacity to stimulate the immune system is retained.
Because the weakened organism can multiply to a limited extent, it closely resembles a natural infection. This generally produces a strong and long-lasting immune response.
Live attenuation may be achieved through:
- repeated growth of the microorganism under controlled laboratory conditions;
- adaptation to non-human cells or lower temperatures;
- genetic modification;
- removal or alteration of genes responsible for virulence.
The organism remains biologically active but loses most of its ability to cause serious disease.
Mechanism of Immune Response
After administration, the attenuated microorganism enters the body and undergoes limited replication.
This exposes the immune system to several components of the pathogen and activates both major arms of adaptive immunity:
- Humoral immunity, involving the production of antibodies by B cells
- Cell-mediated immunity, involving the activation of T cells
The vaccine also generates memory B cells and memory T cells. These cells allow the immune system to respond rapidly when the person is later exposed to the actual disease-causing microorganism.
Live-attenuated vaccines often produce immunity at the site where natural infection occurs. For example, an orally administered vaccine may stimulate mucosal immunity in the intestine.
Since they closely imitate natural infection, such vaccines generally require fewer doses than many inactivated or subunit vaccines.
Examples
Important live-attenuated vaccines include:
- measles, mumps and rubella vaccine;
- oral polio vaccine;
- Bacillus Calmette–Guérin vaccine against tuberculosis;
- yellow fever vaccine;
- rotavirus vaccine;
- varicella or chickenpox vaccine;
- certain influenza vaccines administered through the nose;
- oral typhoid vaccine;
- QDENGA vaccine against dengue.
Some vaccines contain a weakened form of the original disease-causing organism, while others use a modified viral backbone carrying genetic material from different strains or serotypes.
The route of administration varies. Live vaccines may be given through injection, orally or as a nasal spray, depending on the pathogen and the immune response required.
Advantages and Limitations
Live-attenuated vaccines offer several advantages:
- strong and broad immune response;
- activation of both antibody-mediated and cell-mediated immunity;
- long-lasting protection;
- fewer doses in many cases;
- development of immune memory;
- stimulation of mucosal immunity in some vaccines.
However, they also have important limitations.
Because the microorganism is alive, these vaccines may not be suitable for:
- people with severe immunodeficiency;
- individuals receiving strong immunosuppressive treatment;
- pregnant women, depending on the vaccine;
- persons with a history of severe allergic reaction to a vaccine component.
They usually require careful storage and transportation because heat and light may damage the weakened organism. This makes maintenance of the cold chain essential.
In rare cases, the attenuated organism may cause vaccine-associated disease. Reversion to a more virulent form is extremely uncommon but has historically been associated with certain vaccines, particularly the oral polio vaccine.
Live vaccines may also be affected by circulating antibodies. For example, maternal antibodies in infants can sometimes reduce the effectiveness of a vaccine given too early.
Live-Attenuated and Inactivated Vaccines
| Feature | Live-attenuated vaccine | Inactivated vaccine |
| Nature | Contains a weakened living organism | Contains a killed organism |
| Replication in body | Limited replication occurs | Cannot replicate |
| Immune response | Strong and broad | Mainly antibody-based |
| Number of doses | Usually fewer | Often requires multiple doses and boosters |
| Duration of protection | Generally long-lasting | May decline over time |
| Use in immunocompromised persons | Usually avoided | Generally safer |
| Storage requirements | More sensitive to heat | Usually more stable |
| Risk of causing disease | Very low but possible in vulnerable persons | Cannot cause infection from the killed organism |
The choice between live and inactivated vaccines depends on the nature of the pathogen, the recipient’s health, required duration of protection, storage conditions and public-health objectives.
Conclusion
Live-attenuated vaccines are highly effective because they imitate natural infection and generate strong, durable immunity. Their use, however, requires careful screening, proper storage and special precautions for pregnant and immunocompromised individuals.
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