Dear Tad: No, the FeLV vaccine will not cause cats to test positive on an ELISA...both the ELISA and IFA test for presence of antigen from replicating virus, with the former test being 100 times more sensitive than the latter so it will detect very early stages of infection which may be transient, as well as the later stages. An IFA+ is consistent with stage 4-6 of infection.
Vaccinating a cat who has a latent infection can trigger such an infection into an active one. It is more likely that a "faint" positive indicates exposure/very early stage of infection and there is a good chance these kittys could clear the virus if they are put on a healthy diet and given immune support. An IFA test done at this point, if it were negative, could confirm this. An IFA positive would indicate the cats are at least at stage 4 of infection and if it is very early stage 4, the cats could still have a chance to seroconvert, but this would be less likely. An IFA positive is more likely to indicate a cat will remain persistently viremic. Sally in San Jose
