I wonder why annually. I don't think the vaccine changes like the human flu vaccine. This is something I would like to get for Renji since we are in a high risk area but it's always bundled up with six other vaxes and near impossible to find separately.
I believe that there are separate Lepto vaccines, but you might have to order them separately, and work this out with your vet. The separate Lepto vaccines, IIRC, also contain the most serovars and are likely the most protective.
Lepto is a spirochete, like Lyme, and this may be why the lepto vax appears to have a high rate of adverse reactions. (In fact, on some of the older lyme tests, lyme and lepto will both give a positive.)
The reason that it's annual is because it's a bacterial vaccine. In general, only the viral vaccines provide long term immunity. However, with the Lyme vaccine, I know they have measured dogs whose immunity waned by 6 months, and the record is as long as 10 years. So, bacterial vaccines have to be boostered, and they tend also to be less effective. With lepto in particular, many times the dog comes down with a serovar that was not included in the vaccine, so if there is an outbreak, it's important to ask which serovar, since vaccinating with the wrong one will not provide protection, and, in some dogs, they will react to the vaccine.