VACCINATION OF CHICKS WITH LIVE ATTENUATED SUBTYPE B AVIAN METAPNEUMOVIRUS VACCINES: PROTECTION AGAINST CHALLENGE AND IMMUNE RESPONSES CAN BE UNRELATED TO VACCINE DOSE
Abstract Live attenuated subtype B avian metapneumovirus (aMPV) vaccines from two different commercial sources (Vac 1 and Vac 2) were used to vaccinate two groups of day-old specific-pathogen-free chicks. The chicks were challenged at 21 or 49 days of age with a virulent subtype B virus. Parameters compared were persistence of vaccine viruses, their ability to induce humoral antibody responses, and the protection they offered against virulent challenge. Vac 1 virus was detectable for at least 14 days after vaccination, and Vac 2 virus was detectable for at least 7 days after vaccination. The serologic response after Vac 1 by using a subtype B-based enzyme-linked immunosorbent assay was significantly higher than that induced by Vac 2, even though the titer of the recommended dose had a higher titer than Vac 1. After challenge at 21 or 49 days of age, both vaccines gave 100% protection against clinical disease. For the recovery of challenge virus, one of 10 chicks given the Vac 2 was positive by virus isola...