Hi,
less than 5 % of dogs fail the rabies blood test after on injection, so the chances are your dog only needs one then blood test 3-4 weeks later. The dogs that seem more likely to fail are either very young/old or have had recent infection or ongoing other health issues. If your dog is in this latter category, a second rabies jab makes it more likely to pass the blood test which is expensive and you don't really want to repeat that bit!