Key Ideas to Consider When You Make Your Recommendation

By Greg Edwards, D.V.M., Veterinary Operations, Pfizer Animal Health

The marketplace offers many options when you choose vaccines. Some are formulated by companies that do their own research, then develop them. Other vaccines are generics; they may have the same viruses listed, and use nearly identical USDA labels.

We'll take up some USDA labeling questions in another article, later on. The goal here is simply to help clarify the differences among vaccines. Put more plainly: Why should you choose one vaccine over the other?

While this article does not try to fully detail all the differences, it does provide a practical summary of those differences, and suggests some points to consider when you recommend a specific vaccine.

Differences can occur in several areas of vaccine development. In fact, it's possible for two vaccines that serve the same purpose to differ in each one of those areas -- and thus, to provide very different protection for the same pathogen.

Vaccine development and manufacturing begins with a single virus strain. That's the first difference. It's important to know whether the strain or virus that is chosen represents those that create true disease in cattle.

The next difference may be in cell line growth. Once the strain is chosen it must be grown on cells from a species or type of cell not naturally affected by the disease. For example, the 'modified live virus' for a bovine virus affecting lung tissue may have been created using equine kidney cells. The cell line that is chosen can make a difference in the virulence, safety and efficacy of the resulting 'modified' virus.

After the virus is modified, it has to be grown on media -- to multiply its numbers. The choice of media can also affect the antigenicity of the virus.

Another very critical difference is number of passages. To make a vaccine safe – incapable of producing disease -- it must be 'passed' through the above procedure to modify viral cell characteristics many times. The virus must be modified adequately; so, the more times you 'pass' a virus, the safer it becomes.

On the other hand, the more times it is 'passed' the less efficacious it may become. That is, safer -- but less protective. The best vaccines have the correct balance of safety and efficacy.

A vaccine may be low passage or high passage. Pfizer follows the low passage model. We titrate the passages by determining the lowest number of passages needed to attain safety and maximize efficacy. Pfizer actually takes the vaccine in development and tests it on susceptible animals. Then it may be returned for more passages, or go to full development.

When the point is reached where safety is no longer an issue, the minimum number of passages creates the greatest possible efficacy. This is called 'passage beyond virulence.' In contrast, another company might utilize high passage, which is less expensive than titrating the number of passages needed. This can result in a virus that's safe, but also less efficacious in protecting against disease -- because it is modified from the original virus so much more.

Another difference is the time at which the modified viruses are harvested from the media. If the maturity of the viral growth on media is not known, the effectiveness of the vaccine may not be optimum.

For example, when Mannheimia hemolytica is harvested and there are only cell walls and no leukotoxin, the vaccine will be less effective. This end point is not easy to determine. This is the reason why some autogenous vaccine serials are ineffective while others are not.

The last difference in MLV vaccines is the amount of antigen in the vaccine. A vaccine gets its label approval from USDA based on the lowest level of antigen needed to create the protection. This is known as the Minimum Immunizing Dose (MID). Another way to understand this is: What amount of virus must be present at the expiration date? Some vaccines may only include the minimum requirement. Others might have far more.

Not all of these parameters are easy to determine, and they certainly are not found on the label. You will need to do more research to determine some of these criteria.

In conclusion, there may be variations in any or all of these parameters that create differences in the finished product – and, ultimately, in the efficacy of the vaccine.