Clinical

RNA vaccines: the way of the future?

Have you heard? RNA vaccines are now available for dogs and cats. Learn more about the makeup of these vaccines, how they stimulate immunity, and what is known so far about safety and efficacy.

Emily Singler, VMD

Published Apr 3, 2025

When you think of RNA vaccines, do you think Covid-19? That would make a lot of sense, since that is one of the first instances in which RNA vaccines were widely used. Now, with the introduction of RNA vaccines for the prevention of canine influenza and feline leukemia from Merck Animal Health, that reality is changing.

Merck Animal Health announced the launch of these vaccines, Nobivac® NXT Canine Flu H3N2 and Nobivac® NXT FeLV, both of which rely on RNA particle technology, in 2024. Both vaccines are low volume (with a 0.5 mL dose), adjuvant-free, and preservative-free.

Understanding RNA vaccines

John Ellis, DVM, PhD, DACVP, DACVM, Professor of Veterinary Microbiology at Western College of Veterinary Medicine at the University of Saskatchewan, lent his expertise to help us understand the mechanism of action of these new vaccines. “RNA vaccine technology isn’t a new thing,” he said, although it is only recently that we are seeing commercially available applications of it in veterinary medicine.

Ellis explained that the Nobivac NXT Canine Flu and Nobivac NXT FeLV vaccines utilize a small dose of self-amplifying mRNA that codes for antigens from the virus in question along with an RNA-dependent RNA polymerase obtained from a Venezuelan equine encephalitis virus. This enzyme produces additional copies of the mRNA inside of dendritic cells in the vaccinated animal. After the mRNA has been amplified inside the dendritic cells, ribosomes read out the mRNA and translate it into proteins that would typically be found on the surface of the virus in question (either the canine influenza or feline leukemia virus).

Safety

Ellis identified several attributes of this RNA particle technology that, in theory, should reduce the risk of adverse vaccine reactions when compared to other types of vaccines. Since the mRNA is amplified in the host cells, only a small dose of mRNA (in comparison to other mRNA vaccines like the initial Covid-19 vaccines) is included in the vaccine itself. This smaller dose should correlate with a lower risk of reactions, he said. The absence of an adjuvant also should reduce the risk of adverse effects, he added, “but I don’t think we know until it gets out into a large population of dogs.”

At this time, the available USDA safety data for the NXT Canine Flu vacc H3N2 vaccine noted a 2.5% rate of vaccine adverse events among 654 dogs who were each given two doses of the vaccine (a total of 1301 doses) , including lethargy (1.6%), diarrhea, (0.3%), polydipsia (0.2%), anorexia, injection site edema, anaphylaxis, and injection site pain (each at 0.1%).

For the Nobivac NXT FeLV vaccine, USDA safety data noted a 2.7% rate of vaccine adverse events among 837 cats who were each given two doses of the vaccine (a total of 1657 doses), including lethargy (0.9%), injection site pain (1.1%), vomiting (0.5%), anorexia (0.2%), and general pain (0.1%).

Efficacy

Before these RNA vaccines were available, modified live vaccines generally produced the broadest type of immune response, Ellis said. RNA particle vaccines, he explained, should produce an even broader immune response. “What makes these vaccines potentially better,” Ellis said, “is you get different antigen processing,” referring to the ability of RNA particle vaccines to stimulate both cellular immunity (through the activation of killer T cells) and humoral immunity (by presenting antigens to B cells) in the vaccinated animal.

Although the number of cats in each cohort involved was small (20 cats in both the vaccinate and placebo groups for each of 3 separate studies), initial challenge studies of the Nobivac NXT FeLV showed that 10 to 22% of vaccinated cats became antigenemic after challenge with FeLV versus 85 to 93% of placebo-vaccinated controls between 3 and 12 weeks after challenge.

Nobivac NXT Canine Flu H3N2 data on file with the USDA reported the vaccines’ efficacy in the face of exposure after two vaccine doses of the vaccine given 3 weeks apart to 20 dogs in the vaccinate group and compared to 20 dogs in a placebo-vaccinated group. 26% of vaccinated dogs were found to have some degree of lung consolidation (with severity scores ranging between 0.3 and 1.0) 10 days after challenge with H3N2 influenza versus 90% of placebo-vaccinated controls (with the severity scores ranging from 0.2 to 22.6). The higher lung scores in the control group were correlated with suppurative pneumonia.

Duration of immunity

The Nobivac NXT FeLV vaccine has a published two-year duration of immunity (DOI). The NXT Canine Flu vaccine, on the other hand, has no published DOI. Madeleine Stahl, DVM, Associate Director of Veterinary Insights Medical Affairs for Merck Animal Health, recommended that veterinarians refer to the 2022 AAHA Canine Vaccination Guidelines that recommend two initial boosters 3-4 weeks apart and then yearly revaccination.

When asked if additional studies to determine a DOI for the influenza vaccine are pending, Stahl said, “Our commitment to rigorous scientific evaluation remains a priority, and we will share findings once they are available.”

Are RNA vaccines the future?

Stahl noted that the animal RNA particle vaccine footprint has already expanded beyond Canine Flu and FeLV in Canada, where 3-year Nobivac NXT RNA particle rabies vaccines are already being marketed for dogs and cats. A swine H1 influenza RNA particle vaccine was also launched in 2024 in the U.S.

In addition to the safety and efficacy benefits, Ellis said RNA vaccines are easier to produce than some other types. “Growing viruses in cells is just very tedious,” he said. It’s also advantageous for diseases like influenza, where the virus can mutate easily and often. “This technology allows one to just tweak the gene” in the vaccine, he explained, to induce protection against a newer strain. “I think it probably in many ways is the future.”