Does your dog's vaccination schedule tell you whether they're still protected?

The most useful recent study comes from three veterinary school clinics in the United States. Gonzalez and colleagues, publishing in Veterinary Immunology and Immunopathology in 2023, tested 188 healthy dogs with known vaccination histories for antibodies to all three core viruses, using reference-laboratory methods (virus neutralization and hemagglutination inhibition). No industry funding was declared.

Of those 188, thirteen dogs were five years past their last vaccination. Here is what their blood showed.

Read this carefully. The wider study found that antibody levels for all three viruses fell with time since vaccination and with age. The authors' conclusion was not that boosters are unnecessary. It was that measuring antibody can help decide whether, and which, booster is warranted.

This isn't just one study

Across multiple studies of dogs three or more years past their last core vaccination, a recurring pattern emerges: parvovirus antibody is usually still there; distemper antibody is the one most likely to have faded.

Taken together, the parvovirus finding is the steadier of the two: seropositive in 87%, 93% (positive to all three viruses), 95% and >98% of dogs across four studies of 100 to 1,027 dogs, plus 12 of 13 in the five-year subgroup. Distemper runs the same direction with a much wider spread: 53.9%, 71.5%, 93% and >98% across the same studies, and 8 of 13 at five years.

Antibodies aren't the whole story

A titer measures antibody in the blood on the day of the test. It does not measure memory B cells or cell-mediated immunity, both of which can persist after antibody falls. That is why guideline bodies are careful to say that a negative titer does not reliably predict that a dog is susceptible. It is also why a titer, on its own, cannot settle the question.

Previously vaccinated animals with low antibody have also mounted rapid booster responses (see rabies, below). None of this shows that a negative-titer dog is protected; that is a separate claim and is Not Established.

What controlled challenge studies show

The most direct evidence comes from studies where vaccinated dogs were deliberately exposed to live virus years later. Three manufacturer studies published in 2004–2005, including Gore and colleagues in 2005 (listed in the sources), showed protection against parvovirus, distemper and adenovirus roughly three years after vaccination, which is what current three-year booster labels rest on. They were manufacturer-conducted and have not been independently replicated.

A 2006 review reported challenge protection at seven years, but the underlying trial data was never independently published. Beyond three years, longer protection is suggested by antibody data, not established to the standard used for vaccine labels.

Where titers can be useful, and where they can't

The usefulness of a titer depends on the virus and on where the test is run.

Parvovirus. Strong relationship between antibody and protection. In-clinic test kits perform well for this virus. This is where a titer result is most informative.

Distemper. Useful, but in-clinic kits have shown poor specificity for distemper in some studies, meaning they can call an unprotected dog protected. WSAVA recommends distemper serology be run at a reference laboratory rather than with a point-of-care kit.

Adenovirus. Potentially informative, but the evidence on how long antibody lasts is mixed, and in-clinic kit accuracy is less settled than for parvovirus.

Grades beside this passage describe the evidence that a titer result is informative for that virus, including test-kit accuracy. They are not a grade of the vaccine.

A rabies titer does not replace legally required rabies vaccination anywhere in the United States. Rabies titers are used for international pet travel and certain exposure-management situations, but vaccination requirements are set by law, not by antibody level. If your dog is due for rabies, talk to your veterinarian about the requirements where you live.

If antibody testing can be informative for the other core vaccines, what does the research show for rabies? The sequence the studies describe is this: after vaccination, antibody can decline over time; immune memory may persist; and previously vaccinated dogs can respond strongly to a booster.

Moore and colleagues, in 2015, followed 74 dogs and 33 cats, including animals overdue for vaccination. All reached ≥0.5 IU/mL within 5–15 days of a booster. It is one study, and it measures antibody response, not protection after exposure.

Dodds and colleagues, in 2020, found immune-memory responses detectable more than six years after vaccination, including in dogs whose circulating antibody had fallen very low. Again a single study (the Rabies Challenge Fund research study), and a memory response is not the same outcome as protection on challenge.

The 0.5 IU/mL benchmark used in rabies serology is an internationally used marker of adequate vaccine response. It was not established as a universal threshold above which a dog is proven protected or below which a dog is proven susceptible.

For parvovirus, distemper and adenovirus, the two most-cited veterinary vaccination guidelines both accept that antibody testing has a role. They differ on how routinely it should be used.

The guidelines differ in how strongly they recommend routine use of titers. This is an area of legitimate veterinary discussion, not a settled question. A guideline statement records expert consensus; it is not itself a trial result.