What it is
Canine influenza is caused by two strains, H3N8 and H3N2. Most dogs get a cough, fever and runny nose for a couple of weeks; a small number develop pneumonia. Almost every unvaccinated dog exposed to it becomes infected, which is why it moves through a daycare so fast.
The vaccine
Two doses two to four weeks apart to start, then a yearly booster. Most vets use a combined H3N2/H3N8 product. It takes a couple of weeks after the second dose to protect, so start well before a booking.
Who needs it
Dogs that go to daycare, board, are groomed, compete or visit dog parks, especially in cities where outbreaks have occurred. Your vet will know whether it is circulating locally.
Facilities
Daycares are the most likely to require it, boarding next. Because the first course takes about a month from first dose to protection, it is the vaccine most often missing when someone books at short notice.
Sources
Rules and guidelines change, and businesses set their own policies. Confirm with your vet and with whoever is asking.