Differential effect of maternal antibodies on efficacy of cellular and cell‐free Marek's disease vaccines

The protective efficacy of cell-free and cell-associated turkey herpesvirus (HVT) vaccines against Marek's disease (MD) was determined by a quantitative protective dose 50% (PD(50)) assay in susceptible chicks from HVT-vaccinated, MD-exposed dams or from genetically comparable, isolation-reared, specific pathogen free dams. Comparison within trials showed that cell-associated and cell-free vaccines were equally efficacious in chicks lacking maternal antibodies (median PD(50)s were about 1-4 plaque-forming units (PFU) ). However, in chicks with maternal HVT/MD antibodies, PD(50)values were increased by 2- to 8-fold for cell-associated vaccine and by 15-to 80-fold for cell-free vaccine. The apparent requirement in antibody-positive chicks for greater doses of cell-free than of cell-associated vaccine to give 50% protection may be of practical importance in establishing the optimum number of PFU in a field dose.