Vaccines protect dairy calves from diseases. That’s the short definition. The “long” story of vaccinating calves is that vaccine performance depends on many, highly variable factors.
Modified live vaccines can be effective in building immunity in cattle, because they stimulate an immune response to a variation of disease-causing organisms. But those live organisms can be touchy and temperamental.
Vaccines are intended to prevent disease in cattle and can be highly effective when delivered intact. But many vaccines also are highly fragile, and can be rendered useless by cold, heat or sunlight.