Another note on titers.
Many people seem to believe that good titers = immunity, when that is absolutely incorrect.
Look at HIV: people who have contracted the virus show antibodies in their blood. That's how HIV is diagnosed, by the way. The blood sample is very easily and quickly tested with an ELISA (enzyme-linked immunoabsorbent assay) where the assay solution will change colour if the HIV antibodies are present.
It's then considered "tested positive" and the person is considered doomed.
The titers indicate an activate immune response to the virus, but it certainly does NOT mean that the response will defeat the virus. In HIV, it's clear that it doesn't.
Whenever an organism is exposed to a (pathogenic) virus or bacterium, it will mount an immune response and antibodies appear in very high concentrations very quickly. Sometimes the immune system wins, and the pathogen is defeated, other times, the very high titers are not enough and the organism dies.
The ONLY thing that titers are useful for are to test whether or not a vaccine "takes". If you vaccinate, wait a week or two and detect 0 titers, then you can conclude that the vaccine was ineffective.
If you wait long enough, titers won't even tell you that!
In fact, you could be exposed to a virus, build immunity against it and create a lot of antibodies.
If you aren't exposed to that same virus again for years, your antibody levels will go down very quickly and may even become completely undetectable (titer would read 0).
However, you will still retain those important immune memory cells, and should you ever be exposed to the virus again, those memory cells will immediately reactivate and mount an immune response.
The advantage to having had previous exposure is that the antibody creating process is slow the first time around, usually too slow to prevent you from getting sick - unless your other types of immune responses can overcome the pathogen. The second time around, the memory cells will greatly accelerate the antibody-making process, so you are far less likely to get sick again from the same virus!
Depending on the virus' rate of mutation, you could be immune for life after getting exposed once.
Vaccines supposedly mimick this process, although it's not easy to select the right antigens (molecular antibody targets).
In the end, it just comes down to speed. Previous exposure = a quicker, more efficient response. No previous exposure = same antibody response, but far slower.
How much research has gone into developing an HIV vaccine? Enormous amounts, with incredible funding. And yet, nothing to show for it.
The virus' hypermutator phenotype makes this acquired immunity ineffective because the virus keeps mutating so quickly that the antibodies that worked for the first few weeks become ineffective, and the body constantly has to activate new antibody-producing cells.
Unfortunately, as I stated earlier, this process is very slow and the immune system simply cannot keep up with the mutations of the virus (not to mention that HIV destroys a vital part of the immune system) and is eventually overcome.
Anyway, sorry for the long explanation. I'm just trying to give an idea of how vaccines work exactly, and what titers do and don't indicate.
If titers were the magical solution to finding out immune status, we wouldn't even be having these discussions. It's just not that simple