Lepto is a bacteria spread through the urine of mammals (wolves, coyotes, squirrels, fox, cows, etc). There are over 100 serovars. The vaccine vaccinates against the 4 most common in dogs. Lepto can cause acute renal failure (this is very, very, very bad). Lepto is also zoonotic, meaning that people are susceptible to the bacteria/disease. The vaccine does NOT prevent infection, only the disease.
dogs don't get ARF and die
in theory the dog could get lepto, pee on your hands/mouth/etc. and give you lepto, then you would get ARF and need a kidney transplant
lepto can cause severe vaccine reactions (any vaccine can, but lepto seems to be worse)
so it really depends on your household (toddlers, chemo, etc), your geographic exposure/risk
in summation chat with your vet in your area
(Joe-was that enough info to help?)
"I worry that you don't worry enough"--another worrier aka the Yarn Harlot