Comparison of intramuscular and subcutaneous administration of Marek's disease vaccine

Four trials employing 50% protective dose (PD50) assays were performed to compare the efficacy of Marek's disease vaccine after intramuscular (i.m.) and subcutaneous (s.c.) inoculation of specific-pathogen-free (SPF) chickens. Female chickens were employed in trials 1 and 2. In trials 3 and 4 the results were separately recorded for both sexes. Cell-associated Marek's disease virus serotype 1 vaccines derived from strain CVI-988 were used. Intramuscular administration yielded PD50 estimates which were below the PD50 obtained by the s.c. route in all five pairs of comparable assays. A 4-fold difference of PD50 estimate, in favour of i.m. inoculation, was observed between the two series of female chickens of the trial which most properly fulfilled the requirements for a PD50 assay. By comparing 25 individual pairs of vaccine dose groups, the best protection was observed in 24 groups of chickens which received the vaccine by i.m. route. These data were statistically analysed by comparing the average degrees of protection provided by i.m. or s.c. inoculation. In trials 1 and 2 the i.m. vaccination provided significantly better protection than the s.c. route. Although not significantly different, the differences in trials 3 and 4 were in the same direction. It is recommended that the potency of the minimally acceptable field dose should be increased if s.c. administration becomes the normal practice.