Detection of modified measles and super-spreader using a real-time reverse transcription PCR in the largest measles outbreak, Yamagata, Japan, 2017 in its elimination era

We aimed to verify the effectiveness of real-time reverse transcription (rRT) polymerase chain reaction (PCR) for detecting cases of modified measles (M-Me) and for predicting super-spreader candidates through the experience of a measles outbreak dominated by M-Me in Yamagata, Japan, during March-April 2017. We applied rRT-PCR to specimens from 35 cases of M-Me, nine cases of typical measles (T-Me) and nine cases of prodromal stage of T-Me (P-Me). From rRT-PCR among the M-Me cases, peripheral blood mononuclear cells (PBMC) showed the highest positive rate (80.0%), followed by throat swab (48.6%), urine (33.3%) and serum (3.1%). The negative result of PBMC in M-Me cases was recovered by the result of a throat swab. In specimens of PBMC, throat swab and urine, M-Me group showed the significantly higher cycle of threshold (i.e., lower viral load) in the rRT-PCR than T-Me and P-Me groups, respectively. Furthermore, three super-spreaders in T-Me or P-Me showed an extremely low cycle of threshold in their throat swab specimens. rRT-PCR using PBMC and throat swab might be helpful for clinical management and measles control by certain detection of M-Me cases and by predicting super-spreading events resulting from measles cases with the high viral load.