This article is from the Childhood Vaccinations FAQ, by Lynn Gazis-Sax firstname.lastname@example.org with numerous contributions by others.
Before the development of the rubella vaccine, epidemics used to occur
at irregular intervals in the spring, with major epidemics at 6 to 9
year intervals. (This means that one was just about due when the
vaccine came out in 1969.) There have been no major epidemics since
1969, but the number of cases of rubella and congenital rubella
syndrome increased starting in 1989 (Merck, also California Morbidity
for November 19, 1993). (It was still a small fraction of the
pre-vaccine number, though, see table of disease frequencies in
section 1.) "Serological surveys conducted in the late 1970s and the
1980s indicated that 10 to 25 percent of United States women of
child-bearing age were shown to be susceptible to rubella."
(California Morbidity, November 19, 1993) It now appears to be
declining again: "Following a resurgence of rubella and congenital
rubella syndrome (CRS) during 1989-1991, the reported number of
rubella cases during 1992 and 1993 was the lowest ever recorded."
(MMWR, cited in June 9, 1994 HICNet Medical News Digest.)