Vaccines are not 100% effective, and dogs can still get distemper if they are vaccinated.
Once you have recognized the symptoms in your dog, contact your vet immediately, and separate your sick dog from other pets. When treating your dog, make sure your dog is hydrated. If your dog refuses to drink, you might have to offer them water with a syringe.
Veterinarians sometimes use the Canine Distemper Virus Antigen Test to detect canine distemper.