Question
Live attenuated vaccines
Answer
Live attenuated vaccines contain a weakened (attenuated) form of the live pathogen, which replicates in the host without causing full-blown disease, closely mimicking natural infection and generally inducing robust, durable immunity (both humoral and cell-mediated), often after a single dose or few doses.
Examples: BCG vaccine (live attenuated Mycobacterium bovis), oral polio vaccine (OPV), MMR vaccine (measles, mumps, rubella), varicella vaccine, yellow fever vaccine, oral typhoid vaccine (Ty21a).
Advantages: strong, long-lasting immunity, often with fewer doses required, and induction of mucosal immunity for orally-administered vaccines (e.g., OPV inducing intestinal IgA).
Disadvantages/contraindications: risk of reversion to virulence (very rare, e.g., vaccine-associated paralytic poliomyelitis with OPV); generally contraindicated in pregnancy and significant immunosuppression, given the risk of disseminated infection from the live organism; require careful cold-chain maintenance, as the live organism is more susceptible to inactivation by heat/improper storage.