Abstract

Background/Objective: African swine fever (ASF), caused by African swine fever virus (ASFV), is a highly contagious viral disease affecting domestic pigs and wild boars, causing severe economic losses. Although commercial ASF vaccines have recently been approved in Vietnam, controlling ASF transmission remains challenging. Since injection-based vaccination is impractical for wild boars, oral vaccination is considered essential. This study aimed to evaluate the safety, immunogenicity, protective efficacy, and dose-related outcomes of ASFV-G-ΔI177L/ΔLVR, a live attenuated ASFV vaccine candidate with deletion in the I177L gene and left variable region (LVR), administered orally to convention pigs. Methods: The ASFV-G-ΔI177L/ΔLVR vaccine candidate was orally administered to conventional pigs at three dose levels (102.25, 105.0, and 106.0 TCID50/dose). At 28 days post-vaccination, pigs were challenged intramuscularly with a virulent ASFV field strain at 102.0 HAD50/mL and monitored clinically. Protection was assessed by ASFV-specific antibody responses (p32) and survival following challenge. Results: Oral immunization was well tolerated, with no vaccine-associated clinical signs observed before challenge. Following challenge, vaccinated pigs showed different protective outcomes among the tested dose groups, with survival rates of 1/4 (102.25 TCID50/dose), 4/4 (105.0 TCID50/dose), and 3/4 (106.0 TCID50/dose), respectively. Pigs that succumbed to infection showed neither detectable viremia nor ASFV-specific antibodies before challenge, suggesting incomplete vaccine uptake may have resulted in insufficient immune induction rather than an adverse effect associated with vaccination. In contrast, pigs that seroconverted prior to challenge were fully protected and exhibited lower viral loads than the control animals. Conclusions: ASFV-G-ΔI177L/ΔLVR was well tolerated as an oral live attenuated vaccine candidate and induced protective immunity against virulent ASFV challenge under the present experimental conditions. Notably, complete protection was observed in the 105.0 TCID50/dose group, supporting the potential of this vaccine candidate for oral immunization strategies against ASF. However, the present data do not allow a definitive conclusion regarding the dose–response relationship, and further studies with larger group sizes and field-relevant models are needed to refine dose selection and practical applicability, particularly for wild boar vaccination.

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