Live-attenuated vaccines use a weakened form of the germ that causes a disease. These vaccines mimic a natural infection within the body without causing severe illness. The pathogen is attenuated, or weakened, but remains viable. This weakened yet live form allows the immune system to recognize and respond effectively.
How Live-Attenuated Vaccines Provoke Immunity
Live-attenuated vaccines introduce a weakened virus or bacterium into the body. It undergoes limited replication within host cells. This controlled replication simulates a natural infection without causing full disease. This triggers a comprehensive immune response, involving both innate and adaptive immune systems.
The immune system’s response includes the activation of various immune cells, such as dendritic cells, T-cells, and B-cells. Dendritic cells recognize and process the vaccine virus, presenting its antigens to T-cells. Both CD4+ and CD8+ T-cells play a role, with CD4+ T-cells assisting in B-cell activation, and CD8+ T-cells directly targeting infected cells.
Activated B-cells produce specific antibodies that neutralize the pathogen, preventing further infection. This response also leads to the development of immunological memory, meaning the body “remembers” the pathogen and can mount a rapid defense upon future exposure. This type of vaccine often provides long-lasting, even lifelong, immunity with just one or two doses, unlike inactivated vaccines that typically require multiple doses to achieve similar levels of protection.
Common Examples of Live-Attenuated Vaccines
Several live-attenuated vaccines protect against common infectious diseases. The Measles, Mumps, and Rubella (MMR) vaccine is a prominent example, protecting against these three viral infections. Administered in childhood, it has significantly reduced the incidence of these diseases globally.
The varicella vaccine prevents chickenpox, a highly contagious viral infection, and reduces its complications. The rotavirus vaccine is administered orally to protect infants from severe diarrhea. The intranasal flu vaccine (FluMist) also uses a live-attenuated virus, designed to replicate in the cooler temperatures of the upper respiratory tract to elicit an immune response against influenza.
Considerations for Vulnerable Populations
Live-attenuated vaccines are not recommended for certain groups due to the potential for the weakened pathogen to cause illness in individuals with compromised immune systems. This includes people with conditions such as HIV/AIDS, those undergoing chemotherapy for cancer, or individuals receiving immunosuppressant medications for organ transplants or autoimmune diseases. For these individuals, the attenuated vaccine strain, though weakened, could multiply and cause a more serious infection than in healthy individuals.
Pregnant women are another group for whom live-attenuated vaccines are avoided. There is concern that the live, attenuated virus could cross the placenta and affect the developing fetus. For instance, the MMR and varicella vaccines are contraindicated during pregnancy, even though studies have not shown an increased risk of birth defects or congenital syndromes following inadvertent vaccination.
Healthcare providers advise women to avoid becoming pregnant for a specified period, usually around one month, after receiving certain live-attenuated vaccines. This precaution minimizes any theoretical risk to the fetus. In situations with substantial risk of severe disease, the decision to administer a live vaccine to a vulnerable individual is made with careful consideration of benefits versus potential risks.
Understanding Vaccine Shedding
Vaccine shedding is the release of vaccine-strain viral particles from a vaccinated individual into the environment. It can occur with certain live-attenuated vaccines, such as the rotavirus vaccine (shed in stool) or the intranasal flu vaccine (present in respiratory secretions). Shedding is an expected characteristic of some live vaccines, as their mechanism involves limited replication.
Vaccine shedding does not typically lead to transmission of the vaccine-strain virus to other healthy individuals. When transmission occurs, it is extremely rare, and the weakened nature of the vaccine virus makes it unlikely to cause illness in a healthy contact. For example, documented cases of chickenpox transmission from vaccinated individuals are rare, and usually result in mild or asymptomatic infections.