Efficacy of swine Influenza A virus vaccines on transmission, viral shedding and clinical signs: Systematic review and meta-analysis

Swine influenza A viruses (swIAV) are a major cause of respiratory disease in pigs, and vaccination remains the main control strategy. The objective of this systematic review and meta-analysis was to quantify the efficacy of swIAV vaccines in pigs, regarding viral transmission, viral shedding and clinical signs. Data were extracted from experimental studies involving vaccinated pigs subsequently challenged with swIAV. The review followed PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines. Article selection was performed using two approaches: conventional dual-reviewer screening and ASReview, an artificial intelligence-based tool for systematic reviews. A total of 163 publications met the eligibility criteria, and data were extracted from 146 studies. Meta-analyses were conducted on 11 articles for transmission, 72 for shedding, and 89 for clinical signs. Vaccination, both homologous and heterologous, significantly reduced the median transmission rate (β) and the median reproduction ratio (R) compared to unvaccinated controls. However, the median R-values remained above 1 (4.15 for heterologous and 1.44 for homologous challenges), indicating continued potential for outbreak occurrence. Vaccination also significantly reduced both mean and peak viral shedding, with standardized mean differences of -0.53 and -0.25, respectively. A generalized linear mixed model revealed a strong effect of vaccination under homologous challenge conditions. Meta-regression confirmed the influence of vaccine and challenge type on shedding outcome. No statistically significant difference in body temperature was found between vaccinated and control groups. This review highlights the need for greater harmonization in experimental study design and reporting, to improve the comparability of swIAV vaccine efficacy studies.