How do viral vector vaccines work and what are their advantages and limitations?

Viral vector vaccines use modified viruses to deliver antigen genes into host cells. Mechanism: replication-deficient virus (adenovirus, MVA, VSV) is engineered to carry antigen gene; vector infects cells and expresses antigen; immune system responds to antigen. Types: non-replicating (most common, single round of infection) and replicating (attenuated, amplifies antigen). Advantages: strong cellular and humoral immunity, single-dose potential, established manufacturing, stability at refrigerator temperatures, and effective antigen presentation. Limitations: pre-existing immunity to vector (especially common adenoviruses) can reduce efficacy; anti-vector immunity limits boosting with same vector; rare adverse events (thrombosis with Ad26 vectors); complex manufacturing; and dose limitations. Solutions include using rare serotypes (Ad26) or non-human adenoviruses (chimpanzee). COVID-19 vaccines (AstraZeneca, J&J) demonstrated platform capability.