I'm near the coast in Orange County, CA If you want more specifics as to where I am, PM me. I can tell you exactly where they weren't affected. I know along this stretch of area, many people have had issues with Frontline Plus. Many of us use that one for ticks, just in case, because we do agility in some areas with woods, brush, etc. I think it's pretty rampant when the vets stops carrying it and so did many local stores (all because customers/clients animals were getting fleas and Frontline Plus just wasn't working). As soon as I put on Advantix, my dogs fleas went away. Ditto with others.
I can't tell if they're immune where I live now and where I lived last year, since I've been on Advantix. I just know what I've heard. And yes, I"ve heard many times that fleas can't/won't build up a tolerance.....so either Frontline has been making non-working flea protection, or the fleas built up a tolerance.