Liveattenuated

Live attenuated vaccines use a weakened form of a pathogen that is still capable of limited replication in the host. Because the organism resembles a natural infection, these vaccines often elicit strong, long-lasting immunity that includes both antibody responses and cellular immunity. They are typically administered to healthy individuals and may confer protection with fewer doses than non-replicating vaccines.

Attenuation is achieved by reducing virulence through methods such as serial passage in cell culture or non-human hosts, or by introducing genetic changes. The attenuated organism can replicate temporarily, generating immune memory without causing disease in healthy recipients. Some vaccines are given by mucosal routes (oral or nasal) and can stimulate local mucosal immunity in addition to systemic responses.

Advantages include robust and durable protection, broad immune responses, and often single-dose or dose-sparing schedules. Limitations include a small possibility of disease in vaccine recipients if attenuation is incomplete or individual factors permit replication, a higher risk of adverse events in immunocompromised people, and contraindications during pregnancy. They also require careful cold-chain storage to maintain viability.

Common live attenuated vaccines include measles, mumps, and rubella (MMR); varicella (chickenpox); rotavirus; and some influenza vaccines administered intranasally. In some settings, oral polio vaccine has been used. A few live vaccines may occasionally revert to a more virulent form, though modern strains are designed to minimize this risk.

Safety monitoring and regulatory review follow vaccination programs, with post-licensure surveillance for rare adverse events. Practitioners assess suitability for live vaccines, particularly in pregnant people and those with weakened immune systems.