Immunity levels cannot be demonstrated with Titres, though this does not mean that immunity does not exist.

Many people who are trying to reduce vaccination are interested in using “titers” as a test to measure whether or not their dog is still immune to a disease. They often speak of titers as showing “high” or “low” immunity, or of “having to” re-vaccinate when a titer is low. While there is not a tremendous amount of research on titers in dogs, I think it’s fair to say there is quite a bit of misunderstanding on the part of pet owners, and even many veterinarians, as to what a titer test does or does not tell us.
A “titer” is a measurement of how much antibody to a certain virus (or other antigen) is circulating in the blood at that moment. Titers are usually expressed in a ratio, which is how many times they could dilute the blood until they couldn’t find antibodies anymore. So let’s say they could dilute it two times only and then they didn’t find anymore, that would be a titer of 1:2. If they could dilute it a thousand times before they couldn’t find any antibody, then that would be a titer of 1:1000.

A titer test does not and cannot measure immunity, because immunity to specific viruses is reliant not on antibodies, but on memory cells, which we have no way to measure. Memory cells are what prompt the immune system to create antibodies and dispatch them to an infection caused by the virus it “remembers.” Memory cells don’t need “reminders” in the form of re-vaccination to keep producing antibodies. (Science, 1999; “Immune system’s memory does not need reminders.”) If the animal recently encountered the virus, their level of antibody might be quite high, but that doesn’t mean they are more immune than an animal with a lower titer.