African swine fever (ASF), an economically damaging disease in domestic pigs, has emerged in Vietnam since 2019. Vietnam is the only country granted licenses for developing and commercializing modified live-attenuated vaccines (LAVs) against the highly pathogenic ASF virus (ASFV). The nationwide implementation of LAVs in Vietnam for prophylaxis has likely influenced the viral genetic pool among the swine population. This study highlighted the incursion of a novel ASF vaccine-like variant into a non-vaccinated breeding herd. Retrospective epidemiology suggested a high replacement rate and improper biosecurity measures might introduce the disease into the herd. Affected gilts displayed non-to-mild symptoms, whereas gestational sows experienced reproductive disorders. Remarkably, severe ulcerative dermatitis in udders was observed in lactating sows 1–2 weeks postpartum. The ASF outbreak was significantly associated with reduced reproductive performance compared to the pre-outbreak period (P < 0.001). Genetic analysis revealed several virulence-associated gene deletions and a marker gene presence in the left variable region, consistent with the ASFV-G-∆MGF vaccine strain. Molecular detection and immunohistochemistry indicated viral antigens distributed in macrophage-like cells of the reproductive organs and affected udders. Microscopic findings implied massive necrotizing vasculitis with fibrinoid degeneration compatible with immune complex-induced lesions. In conclusion, naïve sows are highly susceptible to the novel ASF vaccine-like variant than gilts, underscoring improved biosecurity requirements when introducing replacement gilts and monitoring ASF vaccine-like variants.

African swine fever (ASF) is a contagious haemorrhagic disease in pigs and has become endemic in several Vietnam provinces since the first outbreak in 2019. The presence of carriers and the recurrence of disease in the surviving swine herd after an ASF outbreak has not previously been properly evaluated. In this study, pigs naturally infected with an acute form of ASF were allowed to recover from the disease. A serological follow-up was conducted for more than 14 months with 14 convalescent gilts and their offspring. All convalescent animals had long lasting high serum antibody levels without persistent viremia. They also did not excrete virus via nasal discharge post-recovery. These convalescent pigs could partially perform as replacement gilts despite the fact that ASF affected reproductive performance. Here, we confirmed that there were neither the carriers of nor recurrence of disease in the convalescent pigs and their offspring following the outbreak of acute ASF. These findings may facilitate efforts to design a new farming model in ASF endemic provinces in Vietnam where there is a lack of a repopulation strategy due to the limited funding received from the local regulatory authorities.