denice.r - 2/23/2014 16:13 Interesting question. My thought is vaccines are used to stimulate the immune system to produce antibodies to be able to fight off an "infection" should that happen. This will take some time, more like weeks as opposed to a couple days. This is why you revaccinate. To introduce this "foreign" substance again so the body is able to mount a big enough response to fight it off. Now antibiotics work mainly against Bacteria. These clear the body rather quickly which is why just one injection is not usually adequate. There are exceptions to everything. BUT if the calves are not well or running a temp or sick they are not going to be able to mount an appropriate response to the vaccine AND fight the sickness. Giving both things will be further taxing their system. To have the vaccine work I would think it is best to wait till they are feeling better.