Contribution of measles virus IgG antibody avidity assay to the identification of measles cases

Objective: To evaluate the value of measles IgG antibody avidity assay in identifying the measles cases. Methods: Data from the Measles Surveillance Information System was used to collect laboratory confirmed or discarded cases in 2013-2015, and then tracing back the blood specimens from all measles network laboratories in Tianjin. Measles antibody avidity assay was used to detect and to redefine cases from the discarded ones. Results: A total of 326 measles cases including 267 laboratory-confirmed and 59 discarded cases were enrolled into this study, with 92.33% (301/326) of them aged ≥20 years. Result from the measles IgG antibody avidity assay showed that the ratio of high-avidity was 91.23%(52/57) of the discarded cases, which was significantly higher than 66.95% (158/236) of the laboratory confirmed cases (χ2=13.33, P<0.001). According to the case criterion, 15.25% (9/59) of the discarded cases were redefined as measles cases. Eight out of the nine cases were high-avidity with measles containing vaccine (MCV) vaccination history that named as SVF cases. One in nine cases with low-avidity was with typical clinical symptomatic measles but with no vaccination history of MCV. Conclusion: Measles IgG antibody avidity assay could provide reference serological evidence to reduce the error from those discarded cases caused by false negative results on IgM antibody, when diagnosing the measles cases.