The vaccine is only about 70% effective: http://www.gazehound.com/vagabonds-wish-info-on-feline-leukemia/ (I'm not having luck getting the Merck Veterinary Manual to load. :dk: ).
How about if the cats have access to a small outdoor enclosure, but no contact with other cats?.
We don't get our indoor cats vaccinated for FeLV (or anything else after their first round of vaccines and boosters), and we don't have our ferals vaccinated for FeLV.
Well, if he'd recently contracted it, he could test negative now, but it could be "percolating" in his system, and he might test positive for it in six weeks or 2 months or so.Chopper, the stray could bear results for the retrovirus one way, and then later another way.