Effects of campaign for postpartum vaccination on seronegative rate against rubella among Japanese women

Background: Japan experienced two rubella outbreaks in the past decade (2004 and 2012 - 2013), resulting in 10 and 20 infants with congenital rubella syndrome (CRS), respectively. This study was performed to determine whether the seronegative rate was lower in multiparous women than in primiparous women in Japan.Methods: Hemagglutination inhibition (HI) test results during pregnancy were analyzed retrospectively in 11048 primiparous and 9315 multiparous women who gave birth at six hospitals in northern Japan in the 5-year study period (January 2008 through December 2012). Women with HI titer < 1:8 were defined as susceptible to rubella.Results: The seronegative rate was significantly lower in multiparous than primiparous women aged 30 - 31 years (2.3% [22/967] vs. 4.5% [66/1454], P = 0.0036), 36 - 37 years (3.4% [55/1601] vs. 5.7% [79/1389], P = 0.0030), and overall women (3.8% [350/9315] aged 34.7 ± 5.2 vs. 5.4% [597/11048] for 33.2 ± 5.9, P < 0.001). The susceptible fraction size did not differ largely according to hospital, ranging from 3.5% to 6.3%. Those for each year did not change markedly; 4.5% [150/3369], 5.2% [221/4268], 4.4% [195/4412], 4.6% [186/4056], and 4.6% [195/4258] for 2008, 2009, 2010, 2011, and 2012, respectively. Those for teenagers were consistently high: 22.7% [5/22], 20.7% [6/29], 20.6% [7/34], 13.0% [3/23], and 23.5% [4/17] for 2008, 2009, 2010, 2011, and 2012, respectively.Conclusions: The seronegative rate was significantly lower in multiparous than primiparous women. However, Japanese rubella vaccination programs were insufficient to eliminate CRS.