Vaccines are tested on healthy, well nourished animals of the correct age. Calves that are stressed, sick, filled with worms or poorly nourished respond poorly to vaccines.
If they are too young, colostral antibodies can tie up vaccine proteins so that the calf does not need to mount an immune response.
At three months of age, only 40 percent of calves respond to a vaccine. This rises to 80 percent by four months old and 90 percent by six months.
Even healthy, well-fed, older animals sometimes fail to respond to vaccines.
