How Effective are Vaccines Against Infectious Diseases of Cattle?
By Steven E. Wikse, Beef Cattle Clinician and
Extension Veterinarian, Texas A&M University

Just how effective are vaccines against infectious diseases of cattle? Less than 100 percent. Too often we unrealistically expect vaccines against infectious diseases to be 100 percent effective.

Vaccination stimulates an immune reaction that provides protection against disease if the vaccinated animal is later exposed to the infectious agent in the vaccine.

The USDA requires that all vaccines be safe and elicit a measurable immune response, usually production of antibodies in the bloodstream. Unfortunately, a measurable immune response does not always mean the vaccinated animal is protected against disease.
A better estimate of effectiveness would be clinical trials in experimental or natural settings to determine if vaccinated animals are protected against disease after experimental exposure to infectious agents.

The degree of exposure in experimental trials often exceeds natural exposure to produce a large amount of disease in the non-vaccinated animals. This increases the chance of showing a difference in disease between vaccinated and non-vaccinated animals.

Immunologists consider the true measure of a vaccine’s effectiveness to be the amount of reduction in disease in vaccinated animals compared to the unvaccinated animals. This is called the preventable fraction.

The preventable fraction
In a study where 80 percent of non-vaccinated animals abort their calves, compared to 40 percent of vaccinated animals aborting, after being challenged with an infectious agent, the preventable fraction is 50 percent - the vaccine is 50 percent effective.

The best measure of a vaccine’s effectiveness may be to perform clinical trials under natural conditions on a ranch. The literature contains many reports of vaccine field trials. There are pitfalls, however, in conducting on-ranch vaccine studies.

The greatest pitfall is the variation in natural exposure to infectious agents from year to year due to changes in stocking density, weather and introduction of carrier animals.
Infectious diseases often affect ranches in cycles. If the vaccine trial is conducted in a year of low exposure, there will not be enough clinical disease to show a difference between vaccinated and non-vaccinated animals.

Thus, we need on-ranch vaccine trials performed over multiple years to get an accurate picture of how well vaccines work. Numerous field trials are expensive, which adds to the difficulty.

Vaccines only one part
Even though cattle vaccines are only partially effective in preventing disease, we are still successful in controlling infectious diseases on our cattle ranches because vaccines are only one part of infectious disease control programs!

For more than 40 years, our nearly completed national brucellosis eradication program required vaccination of calves with modified-live strain 19 Brucella abortus vaccine, which protected approximately 65 percent of vaccinated animals.

We’ve nearly eradicated brucellosis with a partially effective vaccine because it is only one part of the program. The other parts of the program are surveillance testing of herds, and surveillance of marketed cattle to identify infected herds, followed by complete herd tests and culling of reactors. The brucellosis eradication program would not have been successful if it only relied on the strain 19 vaccine!

A successful beef herd infectious disease prevention program has three parts:
1. Practices that promote a high level of general resistance to diseases.
2. Practices that minimize exposure to infectious agents.
3. A sound vaccination program.

The program may fail if even one part is poorly implemented. The first two parts of the program were discussed in the February 2005 “Ranchers Management Guide” column, “Prevention of Infectious Disease in Beef Cow-Calf Operations: The “`Non-Vaccination Programs”.

A high level of general resistance depends on proper nutrition, control of parasites and minimizing stress. Minimizing exposure to infectious agents is done by avoiding high stocking densities, avoiding purchase of animals that are carriers of unwanted diseases and isolation of sick animals.

Handling
The vaccination program consists of the specific vaccines used and how they are handled and administered, the timing of administration and whether boosters are properly given.

Only use current-dated vaccines that have been properly stored and not exposed to the sun. Closely follow the instructions on the insert to administer proper volumes of vaccine at the recommended locations.

Be sure to use clean syringes free from any trace of soap or chemical disinfectants when using modified-live vaccines.

The timing of the booster dose of vaccine in an initial immunizing series is critical for development of protective immunity, especially for killed virus vaccines. Generally, the booster is recommended three to four weeks later. If the booster of killed virus vaccine is given greater than eight weeks after the primer dose, it may be ineffective and actually act as a new primer dose.

Improper use of killed virus vaccines was identified as a strong risk factor for the severe herd outbreaks of bovine viral diarrhea that Canada experienced in the early 1990s.

Most experts feel that vaccines have a very positive benefit: cost ratio in cattle operations because they do reduce disease incidence; they do reduce the severity of disease in animals that become sick; they do reduce death rates; and they do protect herds against catastrophic disease outbreaks.

So, the next time you hear someone say cattle vaccines are only partially effective, just smile and say, “That’s OK! They are only one part of our ranch’s infectious disease control program!”