Rubella immunity. Defining the level of protective antibody.

The Rubella Subcommittee of the National Committee for Clinical Laboratory Standards has proposed lowering the breakpoint to define rubella immunity from 15 to 10 IU/mL. This recommendation stems from epidemiologic studies on vaccinated persons with low levels of antibody and anecdotal reports. Additional support comes from Centers for Disease Control and Prevention studies and reports. The effectiveness of rubella vaccination is well documented and the 10 IU/mL antibody level is protective in the vast majority of persons. Sporadic reports of viremia and/or reinfection among previously immunized persons with low antibody levels have been reported but proven cases of reinfection have also occurred in persons with titers greater than or equal to the 15 IU/mL cut-off. Despite the occasional occurrence of rubella reinfection in persons with low titers, the theoretical risks are small especially as compared with significantly greater risk in persons who have not been vaccinated. Immunity in a given patient is a clinical decision and the results of antibody tests for rubella, like other laboratory tests, must be evaluated in the context of the clinical setting.