The invention provides a method of priming an immune response in the presence of maternal antibody in a young animal by administering to a 1-90 day old animal, a biocompatible and non-toxic solid phase implant containing an immunogenic agent. A preferred implant according to the invention is one that becomes gradually disintegrated in situ the animal. The implant provides for extended sustained delivery of the immunogenic agent into surrounding tissue fluids in the presence of circulating maternal antibodies to provide a priming dose of the immunogenic agent to stimulate substantially immediate antibody production for active immunity against a pathogen when passive protection is no longer provided by circulating maternal antibodies.