Dr. Patrick Carney explains whether to lump or split multiple vaccines for animals

When an animal is due for multiple vaccines, the question is whether to administer all of them at once or spread them out over several visits. Dr. Patrick Carney, an assistant professor at Cornell University College of Veterinary Medicine, recently discussed this and stated that it is preferable to administer all vaccines at the same time.

Speaking at AVMA Convention 2022 in Philadelphia, Dr. Carney presented “To Lump or Split: When an Animal is Due for Multiple Vaccines”. In his presentation at this convention, he elaborated on both approaches.

Multiple vaccine administration is concerning because it may compromise the immune system's ability to respond adequately to each vaccine. However, in humans, a paper estimated that the theoretical number of vaccines that could be administered concurrently before the immune system is overwhelmed is about 10,000.

Another potential issue with multiple vaccine administration is vaccine antagonism. It occurs when vaccines interact negatively. There are numerous examples of this phenomenon in human vaccinology, but the decrease in immunologic response is clinically insignificant.

In companion animal vaccinology, Dr. Carney discovered only one case of vaccine antagonism. A dog study discovered that when the dogs were also vaccinated against leptospirosis, titers to distemper virus, adenovirus, and parvovirus were reduced one year after vaccination. Nonetheless, the study's findings suggest combining the vaccines reduces the risk of vaccine failure. Vaccine failure is defined as titers below a protective threshold, but that was not the result of lump administration.

Vaccine-associated adverse events (VAAEs) can and do occur in pets. Two extensive studies from Banfield Pet Hospital discovered some VAAEs. For example, administering six vaccines to dogs at the same time more than doubled the risk of an adverse event, while administering five vaccines to cats at the same time more than tripled the risk.

However, the base rate of an adverse event was very low, at 0.25% for dogs and 0.27% for cats. For six vaccines in dogs, the rate of at least one adverse event was 0.56%, or about one in 200. For five cat vaccines, it was 0.83%, which is still less than one in 100. According to probability theory, the risk of VAAEs for a single vaccine administered an equivalent number of times is 1.5% in dogs and 1.3% in cats.

Dr. Carney said, “The one argument for splitting that really holds water for me is that you can tell what caused an adverse reaction when one occurs.”

Nonetheless, he would prefer not to split vaccines for 199 dogs to determine what caused the reaction in the 200th dog. Instead, he'd rather deal with the 200th dog later.

The American Academy of Pediatrics also advocates for a single vaccination schedule for children. In studies, children with a split vaccine schedule have more vaccine failures due to missed doses. Dr. Carney believes that the veterinary situation may worsen, with more missed appointments resulting in more vaccine failures.

Dr. Carney said, “A lot of what I talk about with my students is perception of risk and our inability to really do a good job of assessing risk,” he continued, “The bottom line is that an undervaccinated population is far, far, far more risky from a public health perspective than the vaccines themselves.”