[Ellie May]

Originally Posted by jp4m2

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. 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.

A high titer is strongly correlated with either recent infection or good immunity, but the opposite isn't true. You can use a titer test about two weeks after vaccination to determine if the vaccination was effective in stimulating an immune response (in other words, if the animal was successfully immunized from the vaccine), but testing that same animal's titer years down the road doesn't really tell you anything new.

Very well put.