A recent study of pet cats in Germany suggests many adult cats may not have immunity to feline panleukopenia virus (FPV).

From the study abstract:

Of the 350 cats, 103 (29.4%) had no antibodies against FPV. Chronic kidney disease, neoplasia, glucocorticoid therapy, and vaccination status were significantly associated with a lack of antibodies. The cats with no antibodies were likely to have inadequate immunity against panleukopenia and those with chronic diseases and those receiving glucocorticoids were less protected.

Read more, and purchase the full article, here.

K. Mende, B. Stuetzer, C. Sauter-Louis, T. Homeier, U. Truyen, K. Hartmann, Prevalence of antibodies against feline panleukopenia virus in client-owned cats in Southern Germany, The Veterinary Journal, Available online 7 January 2014, ISSN 1090-0233, http://dx.doi.org/10.1016/j.tvjl.2013.12.023.