Immunization Dose-Dependent In Vivo Replication Kinetics and Protective Efficacy of the Gene-Deleted Marek’s Disease Vaccine SC9-1

In this study, we evaluated the replication kinetics and protective efficacy of the gene-deleted Marek’s disease (MD) vaccine strain SC9-1 in specific-pathogen-free (SPF) chickens immunized with different doses. Chickens were divided into two groups: one group received a high dose (5000 PFU) and the other a low dose (500 PFU) of the vaccine at 1 day of age, with a non-vaccinated group serving as the control. Vaccine viral copy numbers in the spleen and feather tips were detected by quantitative PCR (qPCR) at different days post-vaccination (dpv) (3, 5, 7, 11, and 14 dpv). Meanwhile, a challenge trial was conducted where chickens were challenged with the very virulent Md5 strain (1000 PFU/bird) at 5 dpv and observed until 70 days of age. Results from the vaccination-only experiment revealed that viral replication in the spleen was detectable in 100% of chickens as early as 3 dpv, reaching its peak at 5 dpv, with the high-dose group exhibiting significantly higher viral loads than the low-dose group. In feather tips, replication was detectable in 100% of chickens by 5 dpv and peaked at 7 dpv, but no significant difference was observed between the two dose groups. The unvaccinated control group exhibited 100% mortality with characteristic gross and histopathological lesions of Marek’s disease. In stark contrast, both vaccinated groups (high and low dose) demonstrated 100% survival until the end of the observation period (70 days). No clinical signs or pathological abnormalities were observed in these groups. Furthermore, vaccination with SC9-1 did not adversely impact growth performance and the development of major immune organs, even following subsequent Md5 challenge. The results indicated that a higher initial vaccination dose of the SC9-1 vaccine was associated with elevated peak early in vivo replication level, whereas vaccination dose may not govern viral maturation and shedding levels of SC9-1 in feather-tip tissues. However, peak replication level was not a direct determinant of final protective efficacy. Notably, even the lowest tested dose (500 PFU) afforded full protection against early challenge with a very virulent MDV strain. Collectively, these observations contribute to our understanding of dose-dependent protection induced by gene-deleted MD vaccines and inform rational vaccination strategies.