To analyze the relationship between homologous and heterologous serological titers of immunized pigs and their protection statuses against FMD virus challenges, in the present study, the correlation between the virus neutralization titers at 21 and 28 dpv and the protection statuses at 28 dpv against challenge with FMD virus was analyzed using data sets comprising five different combinations of homologous or heterologous challenge experiments in pigs vaccinated with type O (n = 96), A (n = 69), and Asia 1 (n = 74). As a result, the experiments were divided into three groups (21D-1, 21D-2, and 21D-3) in the 21-dpv model and two groups (28D-1 and 28D-2) in the 28-dpv model. Each response curve of groups 21D-1 and 21D-2 in the 21-dpv model was very similar to each curve of groups 28D-1 and 28D-2 in the 28-dpv model, respectively, even though there was an exceptional extra group (21D-3) in the 21-dpv model. The average titers estimating 0.75 probability of protection ranged from 1.06 to 1.62 log₁₀ in the 21-dpv model and from 1.26 to 1.64 log₁₀ in the 28-dpv model. In summary, we demonstrated that the serological method is useful for predicting the homologous and heterologous protection statuses of vaccinated pigs.

Three commercial vaccines are administered in domestic livestock farms for routine vaccination to aid for foot-and-mouth disease (FMD) control in Korea. Each vaccine contains distinct combinations of inactivated serotype O and A FMD virus (FMDV) antigens: O/Manisa + O/3039 + A/Iraq formulated in a double oil emulsion (DOE), O/Primorsky + A/Zabaikalsky formulated in a DOE, and O/Campos + A/Cruzeiro + A/2001 formulated in a single oil emulsion. Despite the recommendation for a prime-boost vaccination with the same vaccine in fattening pigs, occasional cross-inoculation is inevitable for many reasons, such as lack of compliance with vaccination guidelines, erroneous application, or change in vaccine types by suppliers. Therefore, there have been concerns that a poor immune response could be induced by cross-inoculation due to a failure to boost the immune response. In the present study, it was demonstrated by virus neutralization and ELISA tests that cross-inoculation of pigs with three commercial FMD vaccines does not hamper the immune response against the primary vaccine strains and enhances broader cross-reactivity against heterologous vaccine antigens whether they were applied or not. Therefore, it could be concluded that the cross-inoculation of FMD vaccines can be used as a regimen to strategically overcome the limitation of the antigenic spectrum induced by the original regimen.

It has been reported that a commercial solid-phase competition ELISA (SPCE) commonly used in year-round serum surveillance in South Korea may not be sufficient for use in the sero-monitoring of protective antibody responses following vaccination. Here, we established a liquid-phase blocking ELISA (LPBE) method based on the purified 146S antigen of the foot-and-mouth disease virus (FMDV) A/Yeoncheon/SKR/2017 strain, and its analytical performance was assessed using a wide range of pig sera. The established LPBE performed well regardless of whether antisera were elicited by a homologous (A/Yeoncheon/SKR/2017) or heterologous virus (A/Pocheon/SKR/2010) classified in the A/ASIA/Sea-97 lineage. The LPBE performance was also assessed using sera obtained from pigs in the field that underwent routine vaccination, demonstrating that the assay exhibited higher consistency with the virus neutralization test (standard reference test) than the commercial SPCE. This suggested that the LPBE performed well for antisera induced by the FMDV serotype A vaccine strains used in South Korea, namely, A₂₂/IRQ/24/64, A/Argentina/2001 + A₂₄/Cruzeiro and A/Zabaikalsky/2013. Overall, the established LPBE is considered to be a convenient serological assay for use in postvaccination sero-monitoring.