Abstract
BACKGROUND: African Swine Fever (ASF) is a viral hemorrhagic disease characterized by diverse clinical and pathological manifestations depending on the virulence of isolates/strains and the immunological status of pigs. The use of modified live viruses (MLVs) is currently the most common approach in developing vaccines against ASF. However, despite the availability of dozens of MLV candidates that meet basic safety and efficacy criteria-such as the absence of severe clinical signs and survival after challenge with a virulent strain-no broadly accepted vaccine has yet been developed. Here, we propose viremia testing as an essential criterion for evaluating candidate ASF vaccines, with levels exceeding 10(4) HAD(50)/TCID(50) and lasting longer than 21-28 days post vaccination considered unfavorable indicators. METHODS: We analyzed ASF MLV vaccines obtained through the deletion of one, two, or more genes, focusing on viremia kinetics after vaccination and challenge with virulent ASFV strains. Post mortem data were used to assess viral persistence in organs. RESULTS: Most MLV candidates, especially those with single-gene deletions, demonstrated relatively high viremia levels after vaccination and challenge. Viral persistence was frequently detected in organs upon necropsy. MLVs with an additional EP402R gene deletion showed low viremia after vaccination but high levels after challenge. Nevertheless, several candidates with favorable viremia profiles were identified, including those obtained via targeted deletions or serial passaging in cell cultures. CONCLUSIONS: Incorporating viremia assessment as a primary screening criterion can significantly narrow down the selection of promising MLV candidates and help accelerate the development of effective emergency vaccines for use in ASF-affected regions.