An explanation for the high incidence of congenital rubella syndrome in Ryukyu.

An exceptionally high incidence of congenital rubella syndrome (CRS) has been noted in Ryukan children who were infants in utero during an extensive rubella epidemic in the Ryukyu Islands in 1964 through the spring of 1965. In 1965, the Congenital Rubella Study Group of Kyushu University registered in the Ryukyus 408 cases of CRS and an incidence of 20 cases of CRS per 1000 births. The nearby Amami Islands, on the other hand, had a low incidence of CRS. Seroepidemiologic studies indicated a high prevalence of antibodies in the Ryukyus but a low prevalence in the Amamis. Since CRS is uncommon throughout Japan, some investigators have hypothesized that Japanese strains of the virus are avirulent and that a virulent American strain caused the epidemic in the Ryukyus. However, retrospective seroepidemiologic studies reported here indicate that the high incidence of CRS in the Ryukyus was more probably due to low seropositivity and a resultant high attack rate of rubella among pregnant women, rather than to a hypothetical virulent teratogenic strain of rubella virus. Conversely, the data suggest that the low incidence of CRS in Japan may reflect the infrequency of rubella nonimmunes in women of childbearing age.