The H3N8 vaccine is considered a "lifestyle vaccine" – it's mostly administered to dogs who spend time in group facilities [source: AVMA]. The vaccine won't stop a dog from contracting the flu, but it helps ensure it's the mild form and speeds up recovery. It also reduces the level of contagiousness, so it can help prevent a full-blown outbreak [sources: AVMA, VCA].
The vaccine is only for the original U.S. strain, though. As of 2015, there's no U.S.-approved vaccine for the H3N2 dog-flu virus (though there is one available in South Korea), and it's unclear whether the H3N8 vaccine will work against the new strain [sources: CDC, Iowa State].
Dr. Landolt, for one, is doubtful. "Genetically, the two strains are distinct enough that the vaccine is likely to induce less protection, if any."
Besides vaccinating high-risk dogs, the best approach is to educate yourself and be smart. Talk to your vet about prevention. If dog flu has been an issue in your area, maybe stay away from the dog park for a while. Ask the folks at doggie day care if they've seen any respiratory symptoms in their guests, and if they have, keep your dog home. Make sure any group facility has an infection-control plan in place, which involves checking for respiratory symptoms at admission, immediately isolating a dog that starts coughing, disinfecting everything that dog has come into contact with and notifying all owners of the incident [sources: Iowa State, AVMA].
It's not fool-proof. Dogs are most contagious in the first few days of infection, before symptoms even appear; they can spread the virus for up to two weeks, even after symptoms have disappeared; and about one in five dogs doesn't develop any symptoms at all [sources: VCA, Lewis]. So just because your dog's buddies aren't coughing doesn't mean they don't have the flu.
And if it turns out to be the flu, keep your dog isolated for up to 14 days. Use a short leash on walks and cross the street if another dog is heading toward you (call to the owner "dog flu!" so she doesn't think you just hate her) [sources: Blum Animal Hospital, Chicago Tribune]. Make sure no one gets near his or her food dishes, beds and toys. And then disinfect [sources: Lewis, ASPCA].
Disinfect yourself, too. While your dog is sick, wash your hands and change your clothes before going near another dog [source: Landolt]. Humans can't catch canine influenza, but they can spread it.