What it is
Leptospirosis is a bacterial infection spread in the urine of wildlife and rodents, usually through standing water and wet ground. It can cause kidney and liver failure in dogs, and it can pass to people. Urban dogs are exposed through rats and puddles as readily as rural dogs through streams.
The vaccine
Two doses two to four weeks apart to start, then a booster every year. Protection is against the strains in the vaccine, so it reduces risk rather than eliminating it. It is sometimes combined with DHPP as "DHLPP".
Who should have it
Current guidelines recommend it for most dogs, with the decision made with your vet based on where you live and what your dog does. Dogs that swim, hike, hunt or live where rats are common are the clearest cases.
Kennels
Some boarding facilities now require it, particularly in wet regions. Check the list when you book; it is annual, so it is easy to let lapse.
Lyme disease
Lyme is carried by ticks, mainly in the Northeast, upper Midwest and parts of the Pacific coast. Infected dogs can develop lameness, fever and, rarely, kidney disease. The vaccine is two doses two to four weeks apart and then yearly, and it is recommended for dogs that live in or travel to tick areas. It is never a substitute for tick prevention, which also protects against other tick-borne diseases.
Deciding with your vet
Both are "lifestyle" vaccines in the guidelines: the decision depends on where you live, where you travel and what your dog does. Tell your vet about swimming, hiking, hunting, camping and any trips, and log both in the vaccination record with a reminder, because both are annual.
Sources
Rules and guidelines change, and businesses set their own policies. Confirm with your vet and with whoever is asking.