Again, I would repeat - check with a vet in YOUR area & find out what they recommend.
Lepto is only ONE of the many abortion disease.
And if you give them Lepto, they need a booster shot within so many weeks (check the label) to be effective.
Also, MOST vaccines (even MLV) need a booster for at least the BRSV & Lepto. EVERY vaccine is different, you need to read labels.
I would NEVER go without vaccinating Lepto twice a year (after initial booster) and IBR, PI3, BRSV & BVD, Clostridials & Pasteurella, plus deworm twice a year.
Many old farmers say they don't vaccinate & don't have a problem - but may have unexplained losses.
Prevention is real cheap. Abort one calf & it MORE than pays for the vaccines. Or have one get sick & have to treat with some of the expensive antibiotics.