Conjunctivitis, the key clinical characteristic of adult rubella in Japan during two large outbreaks, 2012-2013 and 2018-2019

Background Rubella virus infection mainly causes illness with mild fever, rash, and lymphadenopathy in children; however, the clinical characteristics of adult rubella are not well-known. Methods An observational study was conducted to compare the characteristics between adult rubella and adult non-rubella among participants aged >= 18 years, with suspected symptomatic rubella. Participants were screened for rubella-specific IgM expression using an enzyme immune assay kit, at a tertiary care hospital in Japan during two outbreaks (January 2012-December 2013 and January 2018-March 2019). Adult rubella diagnosis followed strong positive or paired rubella-specific IgM expression or positive rubella-specific reverse-transcription-polymerase chain reaction. Patients aged <18 years or with clinically suspected rubella with weak or negative IgM expression were excluded. Results Overall, 82 adult rubella and 139 adult non-rubella, with a median age (interquartile range) of 31 (25-41) years and 34 (27-42) years, respectively, were included. Multivariate analysis showed that conjunctivitis (odds ratio 80.6; 95% confidence interval 13.4-486.3; P < 0.001) and male sex (odds ratio 7.1; 95% confidence interval 1.8-28.1; P = 0.005) were significantly associated with adult rubella. Among men born from 1962 to 1979 (high-risk population, n = 68), conjunctivitis also showed a significant association with adult rubella in the multivariate analysis (odds ratio 24.2; 95% confidence interval 1.1-553.7; P = 0.046) as these patients were not included in the national vaccination program. There was no difference in the clinical characteristics between one-time vaccination (n = 11) and no vaccination (n = 8) patient in the adult rubella group. Conclusions Conjunctivitis was the key clinical symptom associated with adult rubella. For the early diagnosis of adult rubella, clinicians should focus on assessing conjunctivitis in patients.