Vaccination of chicks with live attenuated subtype B avian metapneumovirus vaccines: protection against challenge and immune responses can be unrelated to vaccine dose.

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 isolation or reverse transcriptase-polymerase chain reaction (RT-PCR) on each occasion except on day 10 after challenge at 49 days. With groups vaccinated with Vac 1, no challenge virus was detected except on day 5 after challenge at 49 days, when one of 10 chickens was positive only by RT-PCR. This study has shown that commercial aMPV vaccines from different sources can provide identical protection in terms of clinical disease after virulent challenge and in their ability to induce humoral antibodies, even though the titer of their recommended doses is widely different.