From the pups first jab it depends on what the dog has been exposed to .Ive owned terriers for 35 years and killed literally tens of thousands of rats with never a follow up jab and only one instance of lepto in an elderly bitch .
My sister had a jack russel that never saw anything ,was jabbed annually and has recently died with the jaundice that accompanies Lepto at only 6 years old .
There’s a huge mark up on drugs in the UK so why wouldn’t a vet want to jab your dog yearly with something that appears innocuous .
There is no human form of the vaccine .