After reading about Dave's kitty, it got me reading more about FeLV. #5
is particularly interesting to me in the following article.  I'll need to
ask the vet about this when Bonnie's booster is due in May.  The healthy
cats are for the most part separated from Shadow most of the time now,
but it wasn't due to his illness, it is to keep his stress level low.

"FeLV vaccines are intended to protect cats against FeLV infection or, at
least, to prevent persistent viremia. Types of vaccines include killed
whole virus, subunit, and genetically engineered. Vaccines may vary in
protective effect, and manufacturers' claims and independent comparative
studies should be carefully noted. The following guidelines for vaccine
use have been recommended: 1) Only healthy, febrile cats should be
vaccinated. 2) Cats from a high-risk or unknown background should be
tested for FeLV before vaccination. 3) All cats at risk of exposure to
FeLV should be vaccinated. 4) FeLV-positive and FeLV-negative cats should
be kept separated, even if the FeLV-negative cats have been vaccinated.
5) Although it may be preferable to use the same brand of vaccine for
primary inoculations and boosters, use of a different brand for booster
vaccinations should still produce an anamnestic[1] response. This is
because gp70 (SU) is the immunogen in all currently available vaccines.
6) Vaccines that prevent infection entirely are preferable to those that
just prevent persistent viremia. 7) The brand of vaccine should be
selected on the basis of clear research data. The preventable fraction
should be mentioned in the vaccine manufacturers' product information."

<http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/56600.htm>
Under "Prevention and Control"

[1] I had to look this word up but I'm having a hard time putting the
description I read into context. <http://dictionary.reference.com/search?
q=anamnestic>
"Medicine. The complete history recalled and recounted by a patient."