A vaccine-related rash may occur among approximately 5% of recipients of measles-containing vaccine, typically 7-14 days after vaccination. A wild type measles infection and a rash due to vaccination cannot be distinguished by measles-specific IgM testing. In outbreak settings, enhanced surveillance for rash illness may detect suspected cases among potentially exposed individuals who were vaccinated as a control measure. The rapid confirmation of a vaccine-related rash is important to avoid unnecessary public health responses. A vaccine-specific, real-time RT-PCR has been developed to provide a rapid method to identify the vaccine genotype (genotype A) from RNA extracted from clinical specimens collected from suspected cases of measles with a recent history of vaccination prior to rash onset [19] (Annex 6.3).
The real-time RT-PCR that selectively amplifies genotype A, measles vaccine strains (MeVA) includes a probe with four ‘locked’ nucleotides that improve the specificity of the probe. The target for the MeVA assay is the measles N gene, nucleotides 478- 548. Validation studies of the assay have demonstrated that this target region provides an assay that is highly specific for genotype A. The lower limit of detection is about 10X higher than the CDC measles real-time RT-PCR assay. Compared to standard genotyping to determine the genotype and thus discriminate vaccine from wild-type infection, the clinical sensitivity of the MeVA assay is approximately 97%.
Note: When the investigation of a suspected measles case indicates a high probability that the individual may have a rash due to a recent vaccination, the MeVA assay (if available) is performed in parallel with the measles real-time RT-PCR.