Practice question

Major advantage of live attenuated vaccines lies in induction of strong, durable, broad immunity closely mimicking natural infection without causing disease in immunocompetent hosts. Limited replication provides continuous antigen synthesis over days to weeks prolonging immune exposure far beyond bolus injection of killed antigen, sustaining germinal center reactions, affinity maturation and generation of long lived plasma cells producing high affinity IgG persisting decades and memory B cells. Activation of cytosolic innate sensors RIG-I, MDA5, STING by replicating nucleic acids induces robust type I interferon, IL-12, potent CD8 cytotoxic T cell differentiation producing IFN gamma and perforin granzyme killing infected cells, important for viral clearance. Mucosal live vaccines like OPV and rotavirus elicit secretory IgA at portal entry preventing colonization transmission. Single dose often sufficient, sometimes lifelong, reducing programmatic complexity, compliance issues and costs compared to multiple booster schedules required for inactivated vaccines. Examples measles vaccine protection documented more than 20 years, yellow fever 30 years. However safety concern in immunocompromised due to potential progressive infection requires screening contraindications, outweighing advantage in vulnerable populations.