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Pharmaceuticals2026-05-14 · 8 min read

Dog Flu Vaccine: H3N2, H3N8, Outbreak Risk, and What Changed in 2025–2026

Canine influenza vaccine guide — H3N2 vs H3N8 strains, two new 2025 USDA-approved vaccines, outbreak risk, and how the flu shot fits with Bordetella vaccination.

Ran Chen
Ran Chen
Founder, VetMedGuide. Life-sciences operator and 10× global market-access lead.
Published

Canine influenza is caused by influenza A viruses that have adapted to dogs. Two strains have been identified in the United States: H3N8, which jumped from horses to dogs around 2004, and H3N2, which originated in birds and was first detected in U.S. dogs in 2015. Both are highly contagious, both can cause pneumonia, and neither infects humans. The canine influenza vaccine is classified as non-core (lifestyle-based) by AAHA and WSAVA, but it is increasingly relevant as outbreaks continue — particularly H3N2, which is the only strain currently circulating at detectable levels in North America.

H3N8 versus H3N2: what the current landscape looks like

H3N8 was the first canine influenza strain identified in the U.S. It emerged in racing greyhounds in Florida in 2004 and has since been detected sporadically across the country. Cases have declined significantly, and H3N8 is now considered rare or possibly no longer circulating in the U.S. dog population.

H3N2 is the strain driving current outbreaks. It remains the only variant with active circulation in North America. H3N2 has a longer shedding period than H3N8 (up to 24 days in some studies, compared to approximately 7 days for H3N8), which makes it more effective at spreading through congregate dog populations.

A March 2026 advisory from the Los Angeles County Department of Public Health documented that nearly all recent H3N2 outbreaks in LA County were closely linked to dogs imported from other countries. The virus does not appear to be enzootic (permanently circulating) in the local dog population. Instead, it follows a pattern of introduction, rapid transmission in congregate settings, and eventual fade-out. This is important for risk assessment: a dog's exposure risk depends heavily on whether it shares space with dogs that have been recently imported or transported from outbreak areas.

States with confirmed canine influenza cases reported to Merck Animal Health's outbreak tracker in 2025 include California, Illinois, Georgia, South Carolina, Virginia, Maryland, New Jersey, Connecticut, and Vermont. However, surveillance is passive — the map reflects tests submitted, not comprehensive population-level data. The absence of a state from the map does not mean the virus is not present.

What the vaccine does

Canine influenza vaccines are killed (inactivated), adjuvanted products administered by subcutaneous injection. They do not provide sterilizing immunity — a vaccinated dog can still become infected. What they provide:

  • Reduction in the severity and duration of clinical signs.
  • Reduction in viral shedding, lowering transmission risk.
  • Reduction in the probability of progression to pneumonia.

The vaccine requires two doses given 2–4 weeks apart for initial immunization, followed by annual boosters. Dogs as young as 6–8 weeks can receive the first dose.

Two new vaccines approved in 2025

The CIV vaccine landscape changed significantly in 2025 with two USDA approvals:

TruCan Ultra CIV H3N2/H3N8 (Elanco)

Approved by the USDA in July 2025. This is a whole inactivated virus bivalent vaccine covering both H3N2 and H3N8 strains. It uses Elanco's PureFil Technology, a proprietary purification process that removes excess proteins and impurities to reduce adverse reactions. The immunizing dose is 0.5 mL — smaller than traditional CIV vaccines. It is the first vaccine portfolio to receive a Fear Free recommendation. Like other CIV vaccines, it requires two doses 2–4 weeks apart, followed by annual boosters.

Nobivac NXT Canine Flu H3N2 (Merck Animal Health)

This is a self-amplifying RNA (saRNA) vaccine — a newer vaccine platform that stimulates both antibody and cytotoxic T-lymphocyte responses to the H3 hemagglutinin protein of the virus. Unlike traditional killed virus vaccines, saRNA vaccines work by delivering RNA that directs the dog's own cells to produce the target antigen, generating both humoral and cellular immune responses. This product covers H3N2 only.

Both vaccines were evaluated in experimental challenge studies and demonstrated significant disease-sparing responses, measured by reduction in CIV-induced pulmonary lesions.

A peer-reviewed article in Today's Veterinary Practice (March/April 2026) by John Ellis, DVM, PhD, DACVP, DACVM, summarized that both new vaccines "confer similar efficacy" in experimental settings, and that "use of CIV vaccines should be based primarily on assessment of anticipated risk for exposure to circulating CIVs."

Which dogs should receive the flu vaccine

The decision is risk-based. AAHA and WSAVA classify CIV vaccination as non-core. The vaccine is most relevant for dogs that:

  • Board overnight or attend daycare regularly.
  • Visit grooming salons where dogs are held in close proximity.
  • Participate in dog shows, agility trials, or competitions.
  • Are housed in shelters or multi-dog environments.
  • Live in or travel to areas with known active outbreaks.
  • Have comorbidities (cardiac disease, chronic respiratory disease, brachycephalic airway syndrome) that increase the risk of complications from respiratory infection.

For dogs with minimal congregate exposure in areas without active outbreaks, the benefit of vaccination is lower. This should be an annual conversation with your veterinarian, reassessed as the dog's lifestyle and local outbreak status change.

Bivalent versus monovalent: does H3N8 still matter?

Since H3N8 appears to have largely disappeared from the U.S. dog population, an H3N2-only vaccine provides protection against the strain that is actually circulating. However, influenza viruses are unpredictable, and H3N8 could re-emerge. Bivalent vaccines (covering both strains) are available and are a reasonable choice for dogs at high risk — they add insurance without adding extra injections.

The Nobivac NXT product covers H3N2 only; the TruCan Ultra product covers both H3N2 and H3N8. Several older bivalent products also remain on the market.

How the flu vaccine fits with other respiratory vaccines

Canine influenza is one of several respiratory pathogens that circulate in congregate dog settings. The others — Bordetella bronchiseptica, canine parainfluenza virus (CPiV), and canine adenovirus type 2 (CAV-2) — are covered by separate vaccines. Key differences:

Canine influenza vaccine Bordetella vaccine (mucosal)
Pathogen Influenza A virus (H3N2, H3N8) B. bronchiseptica bacteria (and CPiV/CAV-2 in combo products)
Type Killed, adjuvanted, injectable Live avirulent, mucosal (IN or oral) — or killed injectable
Initial series Two doses, 2–4 weeks apart One dose (mucosal) or two doses (injectable)
Booster Annual Annual
Onset of immunity After second dose 48–72 hours (mucosal)

A dog at high respiratory risk may benefit from both. They target different pathogens and are not interchangeable. Bordetella vaccination does not protect against canine influenza, and vice versa.

Outbreak response: what happens when flu hits a facility

When a canine influenza outbreak occurs, it moves fast. The virus has a 2–5 day incubation period, and infected dogs shed virus before showing signs. In a boarding facility or shelter, this means that by the time one dog is diagnosed, multiple dogs have already been exposed.

The LA County Veterinary Public Health advisory from March 2026 recommends:

  • Isolating infected and exposed dogs immediately.
  • Testing dogs with suspected CIV using PCR (nasal and oropharyngeal swabs).
  • Notifying owners of exposed dogs.
  • Avoiding movement of dogs in and out of the facility during an active outbreak.

Vaccination during an active outbreak provides limited immediate protection — the two-dose series takes 2–4 weeks to complete. However, dogs that were previously vaccinated may have milder disease if exposed.

Side effects

Canine influenza vaccines are generally well tolerated. Reported reactions include:

  • Mild lethargy for 24–48 hours.
  • Decreased appetite.
  • Mild fever.
  • Localized swelling or soreness at the injection site.

As with any injectable product, anaphylaxis is possible but rare. Dogs with a history of vaccine reactions should be monitored, and the veterinarian should be informed of any prior reactions before vaccination.

What to ask your veterinarian

  • Are there active canine influenza cases in our area right now? (Check the Merck Animal Health outbreak map for recent surveillance data.)
  • Does my dog's lifestyle put them at meaningful risk for flu exposure?
  • Which CIV vaccine does the practice carry — monovalent H3N2 or bivalent H3N2/H3N8? Is the newer saRNA product available?
  • Should my dog receive both the flu vaccine and the Bordetella vaccine?
  • If my dog is due for boarding next month, is there enough time to complete the two-dose series?

Sources

Topics
Vaccination & Infectious Disease
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