Vaccines for cattle are generally classified as killed or modified-live. When and how vaccines can and should be used changes depending on classification and specific product, so it is critical to read and follow label directions. In general, killed vaccines can be used across all cattle types regardless of previous vaccine history. Most killed vaccines are given in a 2 dose series initially followed by an annual booster. Modified-live viral vaccines have restrictions for use in breeding animals or calves nursing cows; these restrictions also vary depending on previous vaccine history.