Abstract
Background:
Newcastle disease virus (NDV) remains a major threat to poultry despite widespread vaccination. Limitations of conventional vaccines highlight the need for stable oral platforms capable of inducing both systemic and mucosal immunity.

Aim:
This study evaluated a recombinant Saccharomyces cerevisiae–based oral vaccine expressing a multi-epitope HN–F construct for its immunogenicity, protective efficacy, and safety in broiler chickens.

Methods:
A total of 150 Ross 308 broiler chickens were allocated into three groups (n=50/group): yeast vaccine, live attenuated vaccine (LAV), and non-immunized control. Birds were immunized at days 7 and 21. Immune responses were evaluated at defined time points (n=10–12/group). NDV-specific IgY and IgA were quantified by ELISA, hemagglutination inhibition (HI) and virus neutralization assays were performed, and cellular immunity was assessed by flow cytometry and cytokine gene expression. Protective efficacy was evaluated following virulent NDV challenge.

Results:
Yeast-vaccinated birds showed significantly higher IgY levels at day 28 (2.34 ± 0.19 µg/mL) compared to controls (0.41 ± 0.06 µg/mL, p < 0.001), with comparable levels to LAV (2.49 ± 0.22 µg/mL, p > 0.05). Intestinal IgA was significantly elevated (1.95 ± 0.14 µg/mL) versus LAV (1.18 ± 0.11 µg/mL) and control (0.34 ± 0.05 µg/mL, p < 0.001). HI titers exceeded protective thresholds (≥6 log₂). CD4⁺ (29.6 ± 2.2%) and CD8⁺ (21.9 ± 1.8%) T-cell populations were significantly increased compared to controls (p < 0.01). Cytokine expression showed upregulation of IFN-γ (4.7 ± 0.6-fold) and IL-2 (3.8 ± 0.5-fold). Post-challenge survival reached 92% versus 28% in controls (p < 0.001). Viral loads were reduced by >3 log₁₀, and shedding duration decreased to 3–4 days compared to 8–10 days in controls.

Conclusion:
The yeast-based HN–F vaccine induced strong systemic, mucosal, and cellular immunity and provided significant protection against NDV challenge, supporting its potential as a safe and effective oral vaccine platform.

Key words: Hemagglutinin–neuraminidase; multi-epitope vaccine; Newcastle disease; Saccharomyces cerevisiae