Gestational timing and vaccine platform shape maternally derived neutralizing immunity and vaccine RNA detection in piglets following maternal vaccination with C-strain or FlagT4G against classical swine fever.

Control of classical swine fever (CSF) in endemic regions relies heavily on vaccination of breeding herds. Optimizing vaccination strategies is essential for sustainable disease control covering pigs belonging to different categories. Data describing direct comparisons between the classical C-strain and recombinant DIVA-compatible vaccines in the context of gestational timing, maternal antibody transfer, and early postnatal virological outcomes remain very limited. This study evaluated how vaccination in the middle to late pregnancy periods (at 72 days of gestation) with either the live attenuated C-strain or the recombinant FlagT4G vaccine influences early postnatal immunity and molecular detection profiles in piglets. Sow humoral responses induced by vaccination were assessed by E2-specific blocking ELISA and virus neutralization peroxidase-linked assay (NPLA), and piglets were monitored for the presence of maternally derived antibodies as well as vaccine-derived CSFV RNA. Vaccination at 72 days of gestation resulted in limited and transient detection of vaccine-derived RNA in piglets, without showing any clinical signs. Both vaccines induced robust E2-specific binding antibody responses in sows; however, only FlagT4G elicited high-magnitude and broadly cross-neutralizing antibody titres. Interestingly, piglets born from C-strain-vaccinated sows presented E2-specific antibodies detectable by ELISA but neutralizing activity was low and declined rapidly. In contrast, piglets derived from FlagT4G-vaccinated sows displayed stronger, more homogeneous, and functionally relevant neutralizing responses that persisted through the early neonatal period. Notably, following vaccination at 72 days of gestation, the gradual decline of maternally derived neutralizing titres coincided with the typical weaning age (approximately 28-35 days), potentially facilitating timely piglet vaccination with reduced interference. In a second experiment it is shown that vaccination at 44 days of gestation with FlagT4G consistently induced high neutralizing titres in sows and a highly efficient transfer of functional maternally derived immunity to piglets, without detectable vaccine RNA at birth. Overall, gestational timing and vaccine platform critically influenced the magnitude and functional quality of maternally derived immunity. Reliance solely on ELISA-based E2 antibody detection may overestimate the presence of protective immunity, emphasizing that the quantitative and functional assessment of neutralizing responses is essential to optimize maternal immunization strategies and advance sustainable DIVA-compatible CSF control.