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.