Genetic engineering has been used to make vaccines without the virulence gene, e.g. the glycoprotein E gene of certain Aujesky's virus vaccines.

Quality control. Each batch of vaccine is grown up from the same seed stock in the same cell line and tested for infectivity. During vaccine growth it is possible for the virus stock to become contaminated with bacteria or other viruses. For this reason vaccines must be prepared with good sterile technique in virus-free cell line or eggs and especially not in labs where virulent virus is being handled. Early human polio virus vaccines were grown in primary rhesus monkey kidney cells and these were contaminateed with a virus called SV40 which causes tumours in rats but not man, fortunately.   Long-established diploid cell lines are therefore now used for human vaccine production eg rabies.

Live virus vaccines are usually administered under the skin, like inactivated vaccines. They are usually freeze-dried and must be reconstituted without being contaminated with disinfectant. Restrain the animal, tent the skin and do not pass the needle straight through the skin and into your own hand. Other routes include eyedrop, intranasal, skin scarification.