The concern about using modified-live virus (MLV) vaccines in pre-weaned calves is that pregnant cows (the dams of these calves) might abort following exposure to the vaccine virus. This very rarely happens and many thousands of calves receive MLV vaccine prior to weaning without adverse effect to the cow herd. Vaccinating cows prior to breeding, using the same product, further minimizes this risk. However, vaccine manufacturers often warn of the possibility of problems on the label, and if they don't want to assume any liability, then I don't either. Follow the label directions. Recently some MLV vaccines have been labeled to say that it is safe to vaccinate calves if the dams were previously vaccinated with the same product.
The period of viral shedding following MLV vaccination could be many days, so removing the calves from the cow herd on the day of vaccination doesn't avoid the problem. If you have concerns about vaccine virus causing problems when you vaccinate calves prior to weaning then use killed virus vaccines.
