Vaccines are available for chickens, turkeys, and pigeons and are used to induce an antibody response, so vaccinated bids must be exposed to a larger dose of vNDV to be infected. Unfortunately, ND vaccines do not provide sterile immunity, and in many areas of the world vaccines are used to prevent losses from sickness and death. Live lentogenic vaccines, chiefly B1 and LaSota strains, are widely used and typically administered to poultry by mass application in drinking water or by spray. Mucosal immunity induced in birds vaccinated by live vaccines applied by these routes decreases the amount of vNDV the vaccinated birds will shed if infected with vNDV, compared with the immune response induced by an inactivated vaccine. Mass vaccination methods are less labor intensive but if not applied properly may lead to <85% of the flock being immunized, which is needed for herd immunity. Alternatively, individual administration of live vaccines is via the nares or conjunctival sac. Healthy chicks are vaccinated as early as day 1–4 of life. However, delaying vaccination until the second or third week avoids maternal antibody interference with an active immune response. Mycoplasma, some other bacteria, and other viruses affecting the respiratory tract, if present, may act synergistically with some vaccines to aggravate the vaccine reaction after spray administration.
