Detection of NDV-specific antibodies and the level of protection provided by a single vaccination in young chickens

Fourteen groups of young commercial chickens were immunized once with a liv e NDV vaccine using different vaccine doses and routes of vaccination in fi ve experiments. Three to sis weeks later, small groups were selected from e ach flock. Sera were tested by the haemagglutination-inhibition test and a monoclonal antibody blocking ELISA, and the birds were challenged with a vi rulent NDV strain. Degree of protection was dose-dependent in those groups where the vaccine w as administered orally at 3 creeks of age. Aerosol and eye drop vaccination s performed in day-old chicks provided full protection at 5 or 6 weeks of a ge. There was a good agreement bet vc een the two serological methods and p ositive results in any of the tests were reliable forecasts of protection.