After primary antigenic challenge (from natural disease or immunization), IgG avidity is low for about 6-8 weeks. An acute phase serum sample with low avidity IgG can provide confirmation of a suspected case of measles or rubella and support a positive IgM result. However, the possibility of a recent immunization with MR or MMR should be ruled out, since avidity testing does not discriminate between low avidity from wild-type infection or response from a recent vaccination.

The demonstration of high avidity virus-specific IgG antibody in acute serum indicates that a suspected rubella case does not have a primary infection. The most common use for avidity testing is to rule out a primary rubella infection. An avidity test is frequently requested when a positive or equivocal rubella IgM result is obtained following inappropriate testing for rubella IgM from an asymptomatic pregnant woman. High avidity IgG antibody from such cases would be inconsistent with a current or recent rubella infection.

The presence of virus-specific, high avidity IgG indicates a mature immune response from past infection or vaccination. Although reinfections occur, symptomatic reinfections with rubella are extremely rare [14, 15]. Measles reinfections can be symptomatic although the presentation is usually mild, and these cases are detected most often among healthcare workers exposed to an acutely ill measles case. Reinfection cases may also be detected in the context of an outbreak or in household settings involving a confirmed measles case. [16]. Inadequate vaccine potency may be implicated in outbreaks where a large proportion of cases have evidence of previous vaccination and many cases are identified as measles reinfection cases. Refer to Chapter 8. Laboratory testing in support of measles and rubella surveillance in elimination settings for more information regarding the confirmation of measles reinfections.