Characterizing population-level short-term waning of newcastle disease humoral immunity of MDA-positive broiler chicks with day-old HB1 vaccination in Nigeria

Broiler chicks supplied by hatcheries across Nigeria are often vaccinated at day-old with the Hitchner B1 (HB1) vaccine and come from breeder hens with up-to-date Newcastle disease (ND) immunization. In Nigeria, virulent ND virus strains continue to circulate, making it imperative to assess the early population-level anti-ND humoral immune kinetics of broiler birds supplied to poultry farmers. In this study, we employed a hybrid approach integrating serological analysis with quantitative immunokinetic decay modelling to characterize short-term population-level ND humoral immunity waning in broiler chicks sourced from five major Nigerian hatcheries. Findings from this study show a rapid decline in anti-ND antibody levels, with a loss of putative population-level serologic protection observed within the first week of life in MDA-positive broiler chicks with day-old HB1 vaccination. Power-law modelling best captured the short-term non-linear pattern of anti-ND antibody waning in our study, estimating that population-level GMTs remained above the moderately protective threshold for approximately four days of life. These results point to the existence of a potential window of ND susceptibility, indicating an early population-level risk of ND infection for MDA-positive broiler birds under routine day-old HB1 vaccination in Nigeria. Overall, the study highlights the importance of accounting for MDA levels and optimizing vaccination timing to enhance early-life protection against ND in our region.