Assessing herd immunity against rubella in Japan: a retrospective seroepidemiological analysis of age-dependent transmission dynamics

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Abstract
Objective We aimed to epidemiologically assess rubella herd immunity as a function of time, age and gender in Japan, with reference to the recent 2012–2014 rubella epidemic. Design This study is a retrospective seroepidemiological analysis. Main outcome measures The susceptible fraction of the population was examined as a function of age and time. The age at infection was assessed using reported case data. Results Whereas 30 years ago rubella cases were seen only among children, the median (25–75th centiles) age of cases in 2014 was elevated to 32.0 (17.0–42.0) years among males and 27.0 (7.0–37.0) years among females. Susceptible pockets among male birth cohorts 1989–1993 and 1974–1978 were identified, with seropositive proportions of 70.0% and 68.0%, respectively. The majority of female age groups had greater seropositive proportions than the herd immunity threshold, with a minor susceptible pocket for those born from 1989 to 1993 (78.3% seropositive). The age-standardised seronegative proportion decreased to 18.3% (95% CI 16.8% to 19.8%) among males and 15.6% (95% CI 10.0% to 21.2%) among females in 2013, and the immune fraction was not sufficiently below the herd immunity threshold. While the number of live births born to susceptible mothers in 1983 was estimated at 171 876 across Japan, in 2013 it was reduced to 23 698. Conclusions An elevated age at rubella virus infection and the presence of susceptible pockets among adults were observed in Japan. Although, overall, the absolute number of rubella cases has steadily declined in Japan, the elevated age of rubella cases, along with increased numbers of susceptible adults, contributed to the observation of as many as 45 congenital rubella syndrome (CRS) cases, which calls for supplementary vaccination among susceptible adults. Assessing herd immunity is considered essential for routinely monitoring the risk of future rubella epidemics and CRS cases.