Abstract
<jats:p>Vaccinating poultry during an active respiratory outbreak is one of the most consequential and most frequently oversimplified decisions in poultry medicine. The expression vaccination in the face of an outbreak combines biologically distinct interventions: vaccinating clinically affected birds, vaccinating apparently healthy birds in an infected house, and rapidly protecting unaffected houses or farms at imminent risk. Vaccines are not antiviral treatments and cannot reverse established systemic Newcastle disease virus (NDV) dissemination, tracheal ciliostasis, nephritis, or reproductive injury. Their value depends on whether a sufficiently large population remains uninfected long enough to develop useful immunity. This critical narrative review integrates international standards, controlled challenge studies, transmission experiments, vaccine-application research, molecular epidemiology and field observations concerning NDV and infectious bronchitis virus (IBV). Direct evidence for post-exposure vaccination is exceptionally limited. One controlled NDV experiment in broilers found that VG/GA vaccination 24 hours after challenge improved survival and altered shedding, but this narrow experimental interval does not reproduce a clinically recognised, house-wide field outbreak. No comparable controlled evidence was identified showing that live IB vaccination after infection is established improves outcomes in the affected house. Conventional ND vaccines protect more reliably against death than against infection and shedding, and population-level control requires high, uniform immunity. For IB, rapid dissemination, incomplete cross-protection and vaccine-associated respiratory replication make empirical within-house revaccination biologically uncertain. The most defensible use of accelerated vaccination is generally preventive protection of epidemiologically threatened birds not yet infected, subject to product authorisation, diagnosis, prior immunity, application quality and Veterinary Authority requirements. Outbreak vaccination should therefore be treated as a timed epidemiological intervention, not as antiviral treatment or an automatic response to respiratory signs.</jats:p>