It is illegal to vaccinate cattle in the EU as there is no way to tell the difference between an infected animal and a vaccinated animal. To combat the problem Defra have invested £23 million in cattle vaccine and associated diagnostics R&D since 1998, and over the next 4 years have budgeted to spend a further £9.3 million.
Vaccinating badgers is not illegal and there is a badger vaccine available however to vaccinate a significance percentage of the badger population would mean that all badgers in the area would need to be trapped, tagged and injected (tagging required to show who is vaccinate and who is not). Even if the resources to achieve this were available it still does not solve the problem of TB transmission as the badgers that have TB already will not be cured by the vaccine, so they will still be able to pass it to cattle and new (un-vaccinated) badgers. It should be noted that we would be unable to test whether a badger is infected or has been vaccinated. This would mean a programme of continuous vaccination would be required for a long time, and whilst this takes place many thousands of cattle would need to be culled instead.
26,000 cattle were culled in the UK in 2011 to combat the spread of TB. Badgers have been identified as a key vector in TB spread. High incidence areas are to be targeted for a 70% population cull. The randomised badger culling trial showed a decrease in TB incidence within the cull area and only a short lived increase in surrounding area.
The cull is designed to reduce the need to slaughter cattle and to reduce the incidence of TB in the badger population (it has be done successfully in Ireland already, and in New Zealand where a possum cull was undertaken).
As there is no readily available alternative the cull is necessary.