My understanding was that you treated the loft with an antibiotic against paratyphus before vaccinating the lot. This an attempt to clear the problem prior to vaccination. The information I last had to hand was that you had to vaccinate twice a year at six monthly intervals. I have also been told that because there are so many variants that the vaccination is not fool-proof, it is however the only safeguard we have. Antibiotic treatment may clear the birds, but it gives no lasting protection whereas the vaccination does.