Systematic vaccination can be applied when a disease has become enzootic in a country or region. The final goal of the approach is to control or eradicate the disease within the country. This is a long-term vaccination plan that could be applied nationwide to all commercial and backyard poultry. However, after several months of vaccination in enzootic areas, maternally derived antibody (MDA) is present in young chicks, providing some protection and/or interference with vaccination. The aim of this study was to evaluate the level of protection afforded by MDA against challenge with highly pathogenic avian influenza virus (HPAIV), and its suspected interference with current inactivated vaccines in broilers under controlled laboratory conditions. In the first set of experiments, broilers were vaccinated with inactivated vaccines containing H5N2 subtype antigens in the presence or absence of homologue MDAs and challenged with a clade 2.2 H5N1 HPAIV. In the second set of experiments, day-old broilers, either with or without avian influenza MDA, received a regular-type monovalent H5N2 AI vaccine (0.5 ml) or a concentrated (0.2 ml) AI-Newcastle disease virus combined inactivated vaccine subcutaneously. They were then challenged at 11 or 35 days of age. In conclusion, our results indicate that protection induced by day-old administration of inactivated vaccine (regular or concentrated) in the presence or absence of MDA to H5N2 AIV induces poor protection against challenge with H5N1 HPAIV and should not be recommended. Based on our results, vaccination of MDA-positive chickens at a later age (10 days) seems to be a valuable recommendation, although MDAs may still interfere with vaccination to a lesser extent because they are present up to 3 wk posthatch. Therefore, in areas with high infection pressure, when possible, two vaccinations are recommended for optimal protection. Also, it might be advisable to take into account day-old AI MDA titers when one is determining the optimal age of vaccination.
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