Detection of measles virus genome in lymphocytes from asymptomatic healthy children

A total of 342 samples of peripheral blood mononuclear cells (PBMC) were obtained from 145 healthy individuals, which we examined for the presence of measles virus genome RNA by reverse transcription-polymerase chain reaction (RT-PCR), to identify whether asymptomatic infection of measles virus has occurred in healthy children. Measles virus genome was detected in 11 (23.4%) of 47 nonimmunized individuals; all positives for RT-PCR were infants who experienced measles exposure. No genome was detected in those without measles exposure. In 83 individuals immunized with measles vaccine, the vaccine strain genome was detected in 10 (71.4%) of 14 recipients whose PBMC were obtained within 2 months of vaccination. Measles wild-type genome was detected in 36 (46.2%) of 78 individuals, 40 (25.2%) of 159 samples, who had been immunized more than 2 months before. The wild-type measles genome was also detected in 6 (46.2%) of 13 individuals who had been infected with measles in the distant past. The measles PCR-positive rate was not related to the period since immunization or natural infection. Sequence analysis of PCR products demonstrated they were all in the same cluster of D5 lineage, which was the circulating strain during the study period. We obtained 13 samples of nasopharyngeal secretion (NPS) simultaneously from individuals whose PBMC were positive for measles PCR but did not detect virus genome. Measles genome was, however, detected from NPS in cases of acute infection. We conclude that asymptomatic measles infection is common but would rarely become a source of transmission because of negative PCR in NPS.