I think you've misunderstood me. What I wrote was an explanation of one of the reasons farmers are facing financial difficulty in this country which is obviously not a direct justification for killing badgers. Controlling TB is another challenge they face which is perhaps made more urgent in the eyes of the farmers because of the other difficulties they face (poor prices for their meat, rising fuel and feed costs etc etc).
The current test for TB is a bit like the heaf test humans get (the 6 needle thing we all got at school). The cattle are injected into the skin of the neck with a modified form of the bacteria that causes TB and their immune reaction is then assessed by measuring the size of the welt that develops. They then get classed as negative, positive or inconclusive. Any animal testing positive (a "reactor") is destroyed and any herd with positive animals is placed under movement restriction (and therefore cannot send their animals to slaughter or sell them). This can also happen if an animal repeatedly comes up as inconclusive. There is also a blood test for gamma interferon that can be used in conjunction with the skin test in inconclusive animals. The frequency of this routine testing depends on the incidence of TB in the area (ranges from every 1 year to every 4). Once a herd has a reactor they are repeatedly tested, and further reactors are culled until the whole herd tests negative several times before the movement restrictions are lifted.
The problem is that vaccinated animals would also test "positive" with both tests. Therefore if someone was able to fake the ID method chosen to identify vaccinated animals they could pass off reactors as vaccinated and avoid the financial loss that follows having a reactor. I'm not saying farmers would necessarily do this but the system has to be such that this cannot happen. Much as I would love to believe that all farmers are honest, when facing financial ruin by having persistent reactors people will resort to desperate measures, as reported in the article below from the DEFRA website:
"
Evidence is emerging that some cattle farmers in the South West and Midlands may have been illegally swapping cattle ear tags. That means they may have been retaining TB-positive animals in their herds and sending less productive animals to slaughter in their place" http://www.defra.gov...ttle-bovine-tb/
It is not as simple as developing a vaccine (there is one already), they have to develop a vaccine that induces the correct immune response to protect the cattle from TB but one that doesn't react to the modified form of the bug that is used in the skin test. From my basic knowledge of immunology it is my understanding that this is not possible at present.