Exclusive: FOIA Data Reveal Adverse Event Patterns in New RNA Dog Vaccine

Details from a FOIA request to the USDA have revealed severe adverse events following injection of the Nobivac NXT canine flu H3N2 vaccine, the first self-amplifying RNA (saRNA) vaccine widely used in the US for pets. The aftermarket reports received by The HighWire show the first 1,012 pages of 1,888 total pages of adverse events from September 2024 through July 2025. There were 296 cases involving the Nobivac RNA vaccine as a suspect product: 152 were adverse reactions, and 76 were listed as cases of lack of efficacy, meaning the dogs developed respiratory illness or cough after receiving the vaccine.

The dataset shows that there were four canine deaths and one feline death following vaccine administration, but two of the dog cases had significant confounding factors. The other two cases involve dogs that collapsed shortly after receiving the vaccination. In addition, three dogs were euthanized following the vaccine.

-41 cases involved neurological issues

-30 cases of anaphylaxis or hypersensitivity

-52 cases involved vomiting

-19 cases had an injection-site mass/lump/panniculitis/fibrosis/surgery

-26 cases of diarrhea

-5 cases of collapse/shock-type presentations

-4 cases of bloody diarrhea

A 4-year-old golden retriever collapsed 10 minutes after the vaccination, before going into complete cardiac arrest and dying. Merck added anaphylaxis to the report later. A 7-year-old Yorkshire Terrier collapsed 70 minutes after receiving the vaccine, and life-saving care was attempted, but the dog passed away. Diphenhydramine and dexamethasone sodium phosphate were given to the terrier because of previous unspecified vaccine reactions. In addition to the Nobivac flu shot, the 7.33-pound terrier received the Nobivac Canine 1-DAPPvL4 and Nobivac Intra-Trac3 vaccines.

An 8-year-old Shih Tzu was lethargic and vomiting shortly after receiving the vaccination. She was diagnosed with renal failure six days later and euthanized. The event narrative explains that she had received several vaccinations before, including non-Nobivac flu vaccines, but had never received the Nobivac NXT canine influenza vaccine, which is the first approved saRNA vaccine in the country. Merck evaluated this case and said the vaccine was unlikely to be the cause.

saRNA vaccines have limited long-term safety data, much as mRNA vaccines did before they were rolled out to the general public in response to the COVID-19 pandemic. These vaccines are marketed as “innovative, adjuvant-free, non-live vaccines” by Merck.

The first vaccine of its kind was approved by the USDA in 2024 and received limited coverage, but Nicolas Hulscher, MPH of the McCullough Foundation, criticized the lack of safety testing and the manufacturer’s level of transparency.

“It appears that Merck is attempting to camouflage the fact that this product is self-amplifying,” Hulscher wrote. “The primary product description only indicates that it uses ‘revolutionary RNA particle technology.’ However, the novel platform works by RNA particles targeting dendritic cells, where they self-replicate and result in sustained antigen production.“

The vaccine is designed so the RNA can replicate itself through genetic instruction. An saRNA produces the signal inside the body for longer, a design intended to ideally strengthen immunity. Researchers in October 2023 wrote, “many aspects of the interactions between replicons and the host remain insufficiently investigated.” Researchers in April 2025 wrote, “their mechanisms remain not fully understood and warrant further investigation.” The USDA approved the saRNA Nobivac H3N2 vaccine in June 2024.

As with all post-marketing vaccine surveillance, causality cannot be established solely from the reports, and the manufacturer is evaluating whether its product is the cause. In many instances, the dog received multiple vaccines and/or treatments simultaneously.

Some additional reports from the over 1,000 pages received from the USDA so far reveal a large number of dogs had symptoms within hours of the vaccine being administered, including difficulty breathing, seizures, facial swelling, red eyes, itching, vomiting, trembling, hypersalivation, and fever.

A poodle mixed breed puppy was “shivering, crying, inappetent, and had pain at both injection sites” three hours after receiving the Nobivac RNA H3N2 Flu vaccine and the Nobivac 1-DAPPv+L4 combo vaccine in the two different legs. An 8-year-old poodle mix with a history of vaccine reactions developed neurological issues about three hours after vaccination, including wobbliness, weak legs, pain in the hind end, not eating, and sluggishness.

A separate FOIA request to the USDA regarding the trial data used to approve the vaccine confirmed that the safety study included three spreadsheets containing individual animal data, including treatment group, clinical signs, and adverse events. Portions of this data were withheld as “confidential commercial information” provided by Merck. However, the redacted documents still contain adverse-event comments, including references to anaphylaxis, gastrointestinal issues, diarrhea, and decreased appetite.

Nobivac NXT H3N2 is the first widely used saRNA product on the market in the United States. A cat version is also on the market. Merck also received approval for its NXT rabies saRNA vaccine in 2024. The rabies saRNA vaccine has been available in Canada, but is not commercially available in the US due to contractual obligations. It is unknown when it will be adopted by US vet clinics, but it could happen very soon. saRNA is often marketed as “adjuvant-free,” whereas most traditional rabies and influenza vaccines for dogs contain adjuvants to stimulate an immune response.