While I get lots of interesting infectious disease questions every day, most aren’t new.

This one was.

To paraphrase, the question was basically “If we want to minimize the number of injections when vaccinating a dog, can we mix rabies and core (e.g. distemper, parvovirus, adenovirus) vaccines in the same syringe, using rabies vaccine as the diluent for the core vaccine?”

Some animals are hard to inject once, let alone twice. Mixing two vaccines into one syringe would make it a one-shot deal, which has appeal. But are there potential downsides?

There are probably at least a couple of major downsides to consider.

Rabies vaccine is a killed vaccine. “Core” vaccines are infectious (modified live) vaccines. For infectious vaccines to work, the modified (weakened) viruses must still be able to cause a low grade infection, to induce an immune response. If rabies vaccine is mixed with the modified live vaccine, I have no idea what would happen to the modified live organisms. It’s plausible that components of the rabies vaccine (which are not designed to support microbial growth and may contain preservatives to inhibit microbes) could inhibit the core vaccine. So, I wouldn’t have confidence that the core vaccine would work as expected.

Would the rabies vaccine work?

I’m less concerned about the killed rabies vaccine being impacted by the live core vaccine, but the key is I don’t know, and I doubt anyone has any data on that. That creates a couple of different levels of risk.

• If we (veterinarians) use a product in a manner not according to the label, we’re getting into unknown territory. Consider whether the owner would be told that we’re doing something that might inhibit the vaccine(s) from working. We can’t state with confidence that what we’re doing (and charging for) is likely to be effective.

• From a regulatory standpoint, the bigger issue is whether the rabies vaccination would be effective. The expectation is that we are giving rabies vaccine as per the label. If we don’t, but we indicate that the animal is properly vaccinate, that’s dodgy. A rabies vaccination certificate assumes the vaccine was given according to label directions. If the dog was exposed to rabies, it’s far from guaranteed that it would be properly protected, which leads to significant issues with quarantine / confinement periods and managing the risks to the people / animals in contact with the exposed dog.

So, while I completely understand the desire to limit the number of injections, doing something like mixing vaccines in the same vial or syringe is something I’d stay away from.

For cats, there’s an easier solution. There’s a licensed vaccine that includes feline core vaccines and rabies vaccine. For dogs, if we want to give multiple vaccines, we need to give multiple injections to help ensure they’re effective,