6.4 Vaccine-specific RT-PCR for identification of measles vaccine strains

A vaccine-related rash occurs in approximately 5% of recipients of measles-containing vaccines, typically 7–14 days after vaccination. Wild-type measles infection and vaccine-related rash cannot be distinguished using measles-specific IgM testing or standard RT-qPCR, which detects all measles genotypes. In outbreak settings, vaccination campaigns are often implemented to close immunity gaps, particularly following exposure to a confirmed case. Some of these recently vaccinated individuals may subsequently develop rash or other symptoms that are clinically indistinguishable from true measles cases. Moreover, enhanced rash surveillance may identify such suspected cases among recently vaccinated individuals, particularly when vaccination has been used as a control measure. Rapid confirmation of a vaccine-associated rash is therefore essential to avoid unnecessary public health responses. A genotype A-specific RT-qPCR assay (MeVA) has been developed to rapidly and specifically detect measles genotype A strains, which includes all the measles vaccine viruses, in clinical specimens from recently vaccinated individuals who develop rash. This assay may eliminate the need for sequencing in many cases but is much faster than conventional sequencing. Multiple studies have validated MeVA for genotype A detection and demonstrated its ability to distinguish vaccine strains from wild-type viruses in both singleplex and multiplex RT-qPCR formats [23, 24, 25] (Annex A07).