never vaccinate ill pigeons,this will cause a pigeon to become even more ill,and the pigeon wont get enough power to build up a strong immunity against the disease you have vaccinated for,this means the vaccine wont work properly,never vaccinate during an important period,such as racing,breeding,moulting,make sure there is a interval of minimum of 1 month between the date of vaccination and the start of the period, pigeons thats have been vaccinated,should have no contact with other pigeons for a period of 14days, quoted from a vet,