High Concentrations of Measles Neutralizing Antibodies and High-Avidity Measles IgG Accurately Identify Measles Reinfection Cases

Abstract

In the United States, approximately 9% of the measles cases reported from 2012 to 2014 occurred in vaccinated individuals. Laboratory confirmation of measles in vaccinated individuals is challenging since IgM assays can give inconclusive results. Although a positive reverse transcription (RT)-PCR assay result from an appropriately timed specimen can provide confirmation, negative results may not rule out a highly suspicious case. Detection of high-avidity measles IgG in serum samples provides laboratory evidence of a past immunologic response to measles from natural infection or immunization. High concentrations of measles neutralizing antibody have been observed by plaque reduction neutralization (PRN) assays among confirmed measles cases with high-avidity IgG, referred to here as reinfection cases (RICs). In this study, we evaluated the utility of measuring levels of measles neutralizing antibody to distinguish RICs from noncases by receiver operating characteristic curve analysis. Single and paired serum samples with high-avidity measles IgG from suspected measles cases submitted to the CDC for routine surveillance were used for the analysis. The RICs were confirmed by a 4-fold rise in PRN titer or by RT-quantitative PCR (RT-qPCR) assay, while the noncases were negative by both assays. Discrimination accuracy was high with serum samples collected ≥3 days after rash onset (area under the curve, 0.953; 95% confidence interval [CI], 0.854 to 0.993). Measles neutralizing antibody concentrations of ≥40,000 mIU/ml identified RICs with 90% sensitivity (95% CI, 74 to 98%) and 100% specificity (95% CI, 82 to 100%). Therefore, when serological or RT-qPCR results are unavailable or inconclusive, suspected measles cases with high-avidity measles IgG can be confirmed as RICs by measles neutralizing antibody concentrations of ≥40,000 mIU/ml.

FIG 1

Classification scheme of RICs and noncases included in the ROC analysis and derivation of suspected cases evaluated following the ROC analysis. High-avidity measles IgG antibody in serum was the first criterion for inclusion of RICs and noncases in the ROC analysis. The RICs included in the ROC analysis were confirmed by either a 4-fold rise in serum PRN titer or a positive RT-qPCR result, while noncases had no 4-fold rise in serum PRN titer and viral specimens were negative by RT-qPCR. *, measles-specific IgG was detected either by the CDC measles IgG indirect EIA or by a commercially available measles IgG indirect EIA.

FIG 2

ROC curve analyses of concentrations of measles neutralizing antibodies determined by PRN assays of serum specimens from RICs and noncases. The ROC1 analysis was performed with the first (or single) sample collected. The ROC2 analysis was performed with available second samples collected from the same cases ≥3 days after rash onset. The AUC is plotted as a solid line. The diagonal line is an AUC of 0.5, interpreted as a random guess. The 95% CIs are in parentheses and are plotted as dashed lines.

FIG 3

Concentrations of measles neutralizing antibodies determined by PRN assays of serum specimens from RICs and noncases used in the ROC curve analysis. Concentrations (mIU/ml) are shown as box-and-whisker (BW) plots. The whiskers represent the values within the 10th to the 90th percentiles, and the median and 25th and 75th percentiles are depicted by the horizontal lines in the boxes. Individual data points are shown; outliers are shown as black circles, and extreme outliers are shown as triangles (noncases) and squares (RICs). GMCs are shown above the BW plots. The P values shown are for the difference between the GMCs of serum specimens from RICs in ROC1 and ROC2. Statistical significance, P ≤ 0.05. N is the number of samples in each BW plot.

FIG 4

Concentrations of measles neutralizing antibodies determined by PRN assays of serum specimens from two of the RIC patients from whom multiple serum samples were collected. The graph shows concentrations of measles neutralizing antibodies and days of serum sample collection relative to rash onset.