Vaccines for common viral cattle diseases such as respiratory infections or BVD come in two forms: killed vaccines and modified live vaccines.

Killed vaccines don’t contain any live material; they use a protein from the real virus to trigger an immune response. This response may be relatively mild, and a second booster shot is required. Modified live vaccines, however, contain an attenuated version of the live virus and actually replicate in the animal to mimic a real infection.

“A modified live vaccine usually gives a quicker, longer, and stronger response,” says Lee Jones, a veterinarian and beef cow reproductive specialist, University of Georgia. “It can also be mishandled, and, on rare occasions, may cause abortions in cows that were never vaccinated before.”

To know when and how to use each vaccine, he offers this background.


Jones says not all animals respond the same to a vaccine. “Some respond great, some just OK, and some not at all. That’s why we recommend revaccination to catch the 20% or 30% that didn’t respond to the first shot.”

Less-than-perfect vaccine response could be due to the overall health and nutritional status of your herd, he says. Manufacturers test vaccines in ideal conditions on healthy animals. That’s not necessarily the way things are on your ranch. Before a vaccine gets to you, it goes through a lengthy transportation, handling, and storage process.

Stress impacts an animal’s immune response. For instance, calves vaccinated at a sale barn can have a lower response than those vaccinated at home, Jones points out. 

Nutritional status also matters. Research shows good mineral supplementation improves vaccine response by up to 50%. “When a vaccine triggers an immune response, it depletes some nutritional resources in the animal. For a vaccine to work best, the nutrition needs to be high to start with,” Jones says.


Because modified live vaccines create such a strong and lasting immune response, they have advantages. However, they are not without potential problems.

For instance, sometimes pregnant cows never previously vaccinated with a modified live vaccine will abort when vaccinated, perhaps because of a direct effect on the cow’s reproductive tract. “While most cows don’t have any negative effects, it is best to avoid modified live vaccines in pregnant cows that haven’t previously been vaccinated with it,” Jones cautions.

Further complicating this is the fact that there are reports of calves vaccinated with the modified live vaccine actually shedding the virus and infecting their mothers, possibly causing abortions. “This is very rare, and it usually happens because the calves are in poor nutrition or health to start with,” Jones says. “Calves in good condition usually do not shed the virus after vaccination.”

Because of these complications, Jones supports a combination of modified live vaccines and killed vaccines for herd immunity. He says the risk of abortion is lower if a cow had the modified live vaccine at a prebreeding stage in life. That seems to precondition her immune system for a good, safe response to revaccination later.

“Make sure replacement heifers are in good condition, and then vaccinate them with two or three doses of a modified live vaccine before their first breeding season,” he suggests. “That sets them up for good response to a killed vaccine as an annual booster. Any risk of abortions should be greatly reduced.”

Is that combination program good for the life of a cow? “We honestly don’t know,” Jones says. “Everybody has some cows that stick around for a long productive life. We don’t know everything about lifetime immunity.

“What I do believe is that a foundation of a modified live vaccine early in life followed by a killed vaccine is a good program.”