It is a given that false negatives (failing to detect antigen from virus that is actively replicating) to an ELISA test, which is sensitive enough to detect very early stages of infection, are highly unlikely. A "false negative" would more likely be due to error on the part of the technician performing the test.
Cats which initially test positive on an ELISA, but later test negative, have either cleared the virus, which is a likely scenario with a healthy adult cat who becomes exposed and is only transiently infected, but it is also possible that after the initial infection, the virus has become latent, or dormant, and sequestered in the bone marrow. Only a bone marrow reactivation test (and maybe the PCR which tests for the virus in a cat's DNA??) can detect a latent infection. As giving any vaccine is a stress to a cat's immune system, I would not go ahead and vaccinate a cat who was initially positive, then retested negative, without first determining whether or not the cat had cleared the virus; I would want to be sure that the infection had not become latent. There is little point in vaccinating a positive cat for the infecting virus, it is stressing an already-compromised immune system, which could be harmful. It will certainly not provide any protection from a virus which has already infected the cat. It is questionable whether or not an immunocompromised cat can benefit from any vaccines, period. If a cat's immune system has been compromised by a virus like FeLV or FIV, then its body cannot be expected to respond to the vaccination process as would a healthy cat. While it is claimed that it is impossible for a cat to succumb to an illness from a "killed" vaccine, any kind of stress to a compromised immune system can have a negative effect. And if there is enough cumulative stress, opportunistic pathogens can more easily gain a foothold. Sally in San Jose
