Elucidating the Onset of Cross-protective Immunity after Intranasal Vaccination with the Attenuated African Swine Fever Vaccine Candidate BA71ΔCD2

African swine fever (ASF) is a deadly disease of swine currently causing a worldwide pandemic leading to severe economic consequences for the porcine industry. The control of disease spread is hampered by the limitation of available effective vaccines. Live attenuated vaccines (LAVs) are currently the most advanced vaccine prototypes, providing strong protection against ASF. However, the significant advances achieved using LAVs must be complemented with further studies to analyse vaccine-induced immunity. Here we characterized the onset of cross-protective immunity triggered by the LAV candidate BA71ΔCD2. Intranasally vaccinated pigs were challenged with the virulent Georgia 2007/1 strain at days 3, 7 and 12 postvaccination. Only the animals vaccinated 12 days before challenge effectively controlled infection progression, showing low virus loads, minor clinical signs and lack of the unbalanced inflammatory response characteristic of severe disease. Contrarily, animals vaccinated 3 or 7 days before challenge just showed a minor delay of disease progression. Virus-specific antibody responses and whole blood transcriptome signatures demonstrated that control of infection is associated with the presence of virus-specific antibodies and a cytotoxic response before challenge. These results contribute to our understanding of protective immunity induced by LAV-based vaccines, encouraging their use in emergency responses in ASF affected areas. Twenty seven-week-old Landrace x Large white breed male pigs were divided into four groups of five pigs. Three groups were intranasally vaccinated with 10^6 pfu/animal of the attenuated strain BA71ΔCD2 at 3, 7 or 12-days pre-challenge. Each animal received 2 mL of the vaccine diluted in PBS via intranasal inoculation (1 mL/nostril). As a control group, the remaining 5 animals were inoculated with 2 mL of PBS. Each animal was intranasally inoculated with 2 mL containing 10^5 HAU50 of the highly virulent ASFV strain Georgia2007/1 (1 mL/nostril). Clinical signs were monitored daily throughout the length of the experiment. EDTA-blood, sera and nasal swabs were taken the days of vaccination and challenge, and at 3, 7 and 9 days post-challenge (dpc) to assess viral loads and host immune responses. Clinical signs were evaluated following standardized guidelines. Whole blood was collected and analysed at 0, 3 and 7 dpc by RNA-seq.