Comparing crude rates based on less than 10 events

I need to compare crude rates between two areas, and have found it can be done as follows:
D=r1 -r2
where: r1 = rate for County 1 r2 = rate for County 2

CI = D ± square root of (CL12 + CL22)
where: CL1 = confidence limit for County 1 rate CL2 = confidence limit for County 2 rate

and CL = 1.96 x (r / square root of d)
where: d = number of events

However, this approach is limited to cases based upon 10-99 events, some of the rates I need to compare are based on less than 10 events.

Does anyone know what would be the problem comparing those rates as described above?

And does anyone know how to compare two rates where at least one of the rates is based upon less than 10 events?