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Choosing modified-live or killed vaccines for calves

Heather Smith Thomas for Progressive Cattleman Published on 24 January 2019
Giving shots

There are many commercial vaccines available to protect cattle against infectious agents. Some are modified-live vaccines, and others are killed products. It’s not always easy to decide which ones are best when vaccinating calves.

Russ Daly, DVM, South Dakota State University, says some vaccines have to be modified-live, such as intranasal vaccines. “You don’t have a choice, because there are no killed intranasal vaccines. If you are giving toxoids like 7-way or 8-way clostridial vaccines (or C. perfringens C and D toxoid), there are only killed vaccines,” he says.

“Where we do have a choice is for viral diseases like IBR, BVD, BRSV or PI-3. There are some killed and some modified-live vaccines for those. For bacterial pneumonia, and pathogens like mannheimia and pasteurella, there are some live versions of those vaccines as well as killed,” says Daly.

Before a vaccine is approved for sale and use, it must go through testing and approval. The manufacturer must show that it has some benefit and is safe. “The vaccine has to stimulate the immune system. For some vaccines, a lot of research exists to show that this stimulation results in disease reduction in vaccinated animals. For other vaccines, the research may simply show that vaccinated animals have higher antibody titer than unvaccinated animals, and this is enough to get USDA approval,” he explains.

“Any vaccine marketed should be expected to stimulate the immune system if administered appropriately as per label directions. There are several things to consider, however, when trying to decide between modified-live and killed vaccines,” Daly says.


“First, we look at efficacy – what we expect that vaccine to do in the calf. Modified-life vaccines tend to stimulate the immune system in a different way than killed vaccines do. The immune system responds to modified-live vaccines similar to how it responds to the actual viral infection. The immune response is directed more toward cell-mediated immunity, which is very important in protection against viral infections. From an efficacy standpoint, it makes sense to consider using modified-live vaccines when trying to prevent a viral infection,” says Daly.

“This does not mean that killed vaccine can’t do the same thing in terms of prevention and stimulating the immune system, but the body’s response to killed vaccines is more towards antibody production in the animal. Antibodies focus more on pathogens outside body cells, such as bacterial infections or early viral infections. This difference can help us understand which choices we should make regarding effectiveness,” he says.

Donal O’Toole, Ph.D., pathologist at Wyoming State Veterinary Laboratory, University of Wyoming, says his preference for viral agents is to use a modified-live vaccine – when it is safe to use modified-live products.

“If you give vaccine by a natural route, such as intranasal vaccine for respiratory viruses, a modified-live product has advantages. With a killed product, the amount of antigen an animal receives is basically just what is in the injection. By contrast, when using an attenuated modified-live vaccine, the antigen expands in the animal’s body and also moves to some of that agent’s target sites, such as the lungs. It essentially gives animals a mild and generally subclinical infection,” he says.

“Another advantage, especially when using intranasal viral respiratory agents, is that it bypasses the effect of maternal antibodies,” says O’Toole. In young calves, many vaccines do not stimulate strong immune response because calves still have passive immunity from the dam (via antibodies in colostrum).

These antibodies neutralize most or all of the antigens in the vaccine, so the calf’s immune responses are minimal. Most vaccines are ineffective in calves until passive transfer wanes, but intranasal modified-live vaccines can be given at a younger age, with good results.


“The next thing to consider is convenience,” says Daly. “Modified-live vaccines often don’t have strict requirement for boosters, so you can use them in some situations where a killed vaccine might be difficult to implement.” If you only have one opportunity to vaccinate calves and can’t run them through later for a booster, or might be delayed on getting them in again for their next shot, it might be wise to use a modified-live vaccine.

“This last fall, in our part of the country, many producers gave pre-weaning shots at their normal time, but field work and a wet fall delayed weaning. If killed vaccines were used pre-weaning, a booster is critical to provide good immune response. If modified-live vaccine were given, the need for (and timing of) that booster is not as important,” Daly explains. Thus modified-live vaccines provide a little more flexibility.

Another factor is handling and storage requirements of modified-live vaccines, which are less forgiving than killed vaccines. “While both vaccine types must be kept at refrigerator temperatures up to the time of administration, modified-live vaccines must be used promptly after being mixed,” says Daly.


“Killed vaccines, by their nature, can never cause the disease we are trying to prevent,” says Daly. “With live vaccines, though they have been attenuated, there’s a chance the virus can replicate in the animal’s body and cause disease. Approved modified-live vaccines have been tested to ensure that in normal calves this won’t happen. Vaccine companies test vaccine by giving it to calves at a dose several times higher than the label dose, to make sure it can’t revert to the disease form. This means that in most cases, we don’t have to worry about modified-live vaccines causing illness,” Daly says.

This level of comfort may not be present, however, when modified-live vaccines are given to animals that are stressed or have a compromised immune system.

“One concern veterinarians sometimes have about modified-live vaccine safety is when those vaccines are given to pre-weaned calves in close contact with pregnant cows. Unless cows have been well-vaccinated, vaccine companies caution against vaccinating their calves with modified-live vaccines. Otherwise, if shedding occurs from a vaccinated calf, abortions could result,” he explains. These risks are not present when killed vaccines are used.

“This is something a producer should always talk about with their veterinarian, regarding their own operation. This risk might not be an issue if the cows have routinely been vaccinated and have good immunity,” Daly says.


“The final thing to consider is cost. Modified-live vaccines tend to be less expensive than killed vaccines,” says Daly. “Each producer must look at all the pros and cons to determine which vaccine best fits his or her own situation.”  end mark

PHOTO: Modified-live vaccines may be the best choice if you don’t get chances to booster calves. Photo by Heather Thomas.

Heather Thomas is a freelance writer based in Idaho.

Consulting your vet

“It’s not a situation where one type of vaccine is always better than the other. Either type should help stimulate calves’ immune systems,” says Dr. Russ Daly.

Selecting the “best” vaccine is impossible because so many products are available. “Veterinarians can advise you, but their experience varies and they will differ in their advice,” says Dr. Donal O’Toole. “If a veterinarian has a wreck with a certain product, especially if it happens more than once, he or she generally won’t recommend that product again. I teach a course to upper-division veterinary students, and they often want me to tell them which vaccines are the most reliable. Given the large number of licensed products, it’s not that simple,” he says. 

O’Toole says there are probably 90 products available for IBR and another 150 for BVD. “There is no way I can recommend a specific product. Each veterinarian has to try to keep up with what is going on with biological products and use their best judgment. Consult with a veterinarian who knows what diseases circulate in their practice area, and ask which products give the best results over time. It would be nice if we could choose between just five or 10 products with different formulations and strains of the pathogens, but that’s not the case. It’s hard for producers to make these choices, and hard enough for clinicians to try to figure it out. My suggestion is to ask a veterinarian with a few gray hairs, who has seen a lot of things, and go with that advice.”