There is a difference in the effectiveness of vaccines for different diseases. Vaccines against rabies, tetanus, sleeping sickness, and West Nile Virus have proven to be very effective. It is extremely rare to hear of a horse that has contracted any of these diseases that has been vaccinated. Does that mean that the diseases are not present in our environment, or that the vaccines are extremely effective and the horse population continues to be vaccinated appropriately? I think the answer is the latter.
The cases of respiratory viral diseases we see in vaccinated horses are due to the efficiency of the vaccine. This lower efficiency is a result of the rapid changes seen in these viruses that require new viral strains to be included in the vaccine. We saw this situation with this year’s human flu vaccine where the viruses mutated so much the vaccine was minimally effective. We must remember the measure of these respiratory virus vaccines is to establish a good rate of prevention, not complete prevention, of a disease. Also reduction of the severity of the disease must be judged successful for the vaccine.
Consider two other situations seen in human medicine. The first involves West Nile Virus and the second measles. The last several years there have been multiple reports of West Nile Virus in humans. This indicates the virus is present in Colorado. During the same period there have been very few cases in vaccinated horses. We have heard of an increase of measles in children. The measles vaccine has been very effective. The rise in the incidence of measles appears to be due to an increasing number of parents who are not getting their children vaccinated. The disease is apparently still present and is showing up in these unvaccinated children.
Vaccination programs for disease control are predicated on both individual animal immunity and herd or group immunity. If an efficient vaccine protects large numbers of animals or humans, it’s hard for a disease outbreak to get started. Example: West Nile Virus. Conversely, if assume we don’t need to vaccinate our horses because we don’t see cases of a disease, we may risk an outbreak. Example: measles.