I went through the whole thing with Felv with Vixen. She was Felv+. All my other cats were vaccinated against felv and none of my other 4 cats contracted it. I talked to my vet before hand and he said that 10% of cats are immune to the vaccine. About 30% are immune to the disease itself. The vaccine is about 90 - 95% effective in the cats that are not immune to the vaccine. So when you add it all up the effectiveness of the vaccine is very high and it is highly unlikely that a cat that has been vaccinated for felv that lives with a felv+ cat will ever contract it. The virus dies in the open air. And as long as the cats aren't aggressive or intergroom to extremes all should be just fine. Like I said Vixen lived with 4 other cats all of whom were vaccinated for about 3 years before she passed and none of the others were infected.