Session covers whether to lump or split when animal due for multiple vaccines

Updated September 29, 2022

When an animal is due for multiple vaccines, the question is whether to give all the vaccines at one veterinary visit or spread the vaccines among more than one visit. For Dr. Patrick Carney, the answer is clearly to give all the vaccines at once.

Dr. Carney, an assistant professor in community practice at Cornell University College of Veterinary Medicine, discussed both approaches in his presentation “To Lump or Split: When an Animal is Due for Multiple Vaccines” on July 31 at AVMA Convention 2022 in Philadelphia.

A potential concern with administering multiple vaccines is overwhelming the immune system’s ability to respond adequately to each. In humans, however, a paper estimated that the theoretical number of vaccines that can be administered simultaneously before the immune system is overwhelmed is 10,000.

Dr. Meagan Wentworth, then an intern, and Ashley Workman, a licensed veterinary technician, vaccinate a dog at the Society for the Prevention of Cruelty to Animals of Tompkins County, a partner of the Maddie’s Shelter Medicine Program at Cornell University College of Veterinary Medicine. (Photo by Jonathan King/Cornell University)

Another potential concern is vaccine antagonism, in which vaccines interact negatively. There are plenty of examples of this phenomenon in human vaccinology, but the reduction in the immunologic response is clinically irrelevant.

Dr. Carney found only one example of vaccine antagonism in companion animal vaccinology. A study in dogs found that titers to distemper, adenovirus, and parvovirus one year after vaccination were reduced when the dogs had been vaccinated against leptospirosis at the same time. Yet, data from the study suggest that administering the vaccines together actually reduces the risk of vaccine failure, that is, titers below a protective threshold.

Vaccine-associated adverse events can and do occur in pets, of course. Two large studies from Banfield Pet Hospital found that administering six vaccines to dogs simultaneously had more than double the risk of an adverse event, compared with administering one vaccine, and administering five vaccines to cats simultaneously had more than triple the risk.

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

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

Nevertheless, he would rather not split vaccines for 199 dogs just to know what caused the reaction in the 200th dog. He’d rather deal with the 200th dog after the fact.

The American Academy of Pediatrics promotes a lump schedule for vaccination of children. In studies, the group of children with a split vaccine schedule always has more vaccine failures, as a result of missed doses. If anything, Dr. Carney said, the veterinary situation might be worse, with more missed appointments meaning more vaccine failures.

“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,” Dr. Carney said. “The bottom line is that an undervaccinated population is far, far, far more risky from a public health perspective than the vaccines themselves.”

Correction: An earlier version of this article incorrectly identified the location of the photo.