Not long ago, I was walking my pal Buster in my neighborhood. I spotted likely trouble: A Yorkshire Terrier was off-leash in his driveway half a block ahead and across the street. The inattentive owner was busy unloading groceries from the car.

Sure enough, the Yorkie saw Buster when when he and I were directly across the street. The Yorkie ran to us and, luckily, did not get hit by a car. It became clear that he wasn’t coming over to say hi. He lunged for Buster’s throat.

Buster turned at the last instant. The Yorkie fit perfectly in his mouth (Buster is a 65-pound Lab mix). He held the Yorkie for a few seconds. The Yorkie let out a blood-curdling scream and urinated a bit. Buster released, and the Yorkie beat a retreat back across the street and, luckily, didn’t get hit on the way back either.

Thank goodness Buster is gentle. It was clear that the Yorkie, although terrified, was not hurt. I shouted a few angry words about the value of leashes and paying attention. Then I completed the walk. I am sure that neither the Yorkie nor his owner learned any lessons that day.

But a thought crossed my mind: Buster’s rabies vaccine was set to expire in three days. If Buster had hurt that Yorkie, we would have barely slipped in under the wire. Dogs who break skin with their teeth, even if acting in self-defense, are much better off if their rabies vaccines are current. So are their owners, who might face fines and impoundment of their pets in the event of a bite from a non-vaccinated dog.

It had been almost three years since Buster had received any vaccines. There was no question about updating his rabies vaccine. Rabies vaccines are required by law every three years where I live. But what about the other vaccines? Buster hadn’t had a distemper/parvo shot for three years. He hadn’t had a leptospirosis shot for just as long. And it had been more like five years since his last Bordetella vaccine.

I took Buster with me on my next work shift, and I gave him a rabies vaccine. And no others.

In the old days, vets advised annual vaccination for all dogs. However, recent evidence has indicated that such a schedule is overkill. Most reputable outfits (such as the American Animal Hospital Association and many veterinary schools) now advocate three-year vaccination schedules at most. But is vaccination every three years also overkill?

The short answer is: Nobody knows. When it comes to vaccines and their frequency, there is no clarity. I therefore apologize in advance to people who want clear-cut answers, because when it comes to vaccines, there are none. Anyone who claims otherwise is either ignorant or a charlatan.

There are, however, some reliable guiding principles about vaccines in dogs. Your best option is to use them, along with a comprehensive discussion with your vet, to tailor a vaccination schedule that is individualized for your dog. The issue of vaccination frequency is a complex one, and owners should be actively involved in choosing which — if any — vaccines their dogs receive during routine veterinary check-ups.

Here are nine things to consider when planning dog vaccinations:

1. Younger dogs need vaccines more than older dogs

Puppy vaccines are exceptionally important, and unvaccinated puppies contract life-threatening parvovirus at alarmingly high rates. Booster vaccines at one year of age are also important for most dogs. However, as dogs grow older they are less likely to contract diseases like parvo and distemper. I have never seen parvo in an eight-year-old dog. Hence I did not vaccinate Buster (age eight) against parvo.

2. Dogs with different lifestyles need different vaccines

Teacup Poodles in Manhattan generally do not need rattlesnake vaccines, for example. Duck hunting dogs may be at increased risk of leptospirosis as they wade through marshy areas, so the lepto vaccine might be more important for them.

3. Some vaccines are more important than others

Puppies benefit markedly from their DHPP (which includes parvo) shots. Other vaccines, such as the ones for rattlesnakes and Bordetella, are less important and are not appropriate for all dogs. Also, some vaccines are more effective than others. Parvo shots work, period. However, the efficacy of the vaccines for Bordetella and Lyme disease is questioned by many.

4. Some dog vaccines stop diseases that spread to humans

These so-called zoonoses can be serious. Rabies is the most famous example, but leptospirosis (which can cause liver failure, kidney failure, and other nasty problems) is also a zoonotic threat.

5. Rabies vaccination is required by law in most areas

This means that you’ll need to follow the rules set by your municipality, regardless of whether they are in your dog’s best interest.

6. Not every dog needs every vaccine every year

Remember this if you receive annual vaccine reminders from your vet. Those reminders are generated by your vet’s computer system, and they’re merely suggestions. The real vaccination decisions must be made by you, in consultation with your vet.

7. Not all vaccines are completely benign

They’re generally safe, but vaccines are designed to stimulate the immune system, and they can sometimes trigger immune system overactivity and immune-mediated diseases such as hemolytic anemia. Dogs also can have more basic allergic reactions after vaccines. Vaccines may be very rarely linked to cancers (sarcomas in particular) in dogs.

8. Geography makes a difference in vaccination protocols

Hawaii has no rattlesnakes or rabies but lots of leptospirosis. Tucson has lots of rattlesnakes and rabies but less lepto. Michigan has plenty of Lyme disease. There is no point in vaccinating your dog against threats that do not exist where he lives.

9. Every dog has unique reactions and therefore unique needs

Some dogs will develop lifelong immunity after one parvo vaccine. Others require many boosters. There is no perfect way to differentiate which dog is which. Titers have been much bandied as a solution, but they don’t provide a complete picture of the immune system.

In the end, it will be up to you to consider the nuances of these principles and decide which vaccines are appropriate for your dog. A good vet will be willing to work with you to determine which vaccines are appropriate and when.

And what about Buster? When will I vaccinate him again? He’s due for rabies in three years, and that’s when he’ll get it. I doubt that I’ll ever again vaccinate him for Bordetella. I may or may not ever give him another DHPP — I intend to revisit that decision in a couple of years. And I’ll consider vaccination against leptospirosis if an outbreak occurs in our area.

This means that I have to own the risk of Buster contracting parvo or distemper (I’m not worried) or leptospirosis (I’m only a tiny bit worried). If you decide to skip vaccines in your dog, you also will have to take responsibility for that risk. In Buster’s case, I believe the risk is low, and I won’t be losing any sleep.

Other stories by Dr. Eric Barchas:

• The Top 8 Summer Hazards for Dogs
• What’s Euthanasia Like for a Vet? The More I Do, The Harder It Gets
• Is Your Flea-Control Product Hurting Your Dog?

Got a question for Dr. Barchas? Ask our vet in the comments below and your topic might be featured in an upcoming column. (Note that if you have an emergency situation, please see your own vet immediately!)