Factors associated with low immunity to rubella infection on antenatal screening

Rubella infection during the first trimester results in congenital rubella syndrome. There has been little recent published evidence identifying those at-risk of infection in the first trimester of pregnancy. This study examined the level of rubella immunity in pregnant women in a part of Sydney and risk factors for non-immunity.

We looked at data on all confinements at two maternity hospitals in Sydney in the 2-year period between July 1999 and June 2001. Variables included in our data set included mother's country of birth, mother's date of birth, hospital status (public or private patient), parity, rubella status and postpartum rubella vaccination.

Of the 8096 confinements, the mother was documented as being non-immune to rubella in 567 cases (7%) of cases. Of the 567 confinements where rubella status was documented as non-immune, Asian-born women comprised of 65% (366) of non-immune women while 13% (73) were Australian-born. Country of birth remained a strong predictor of immunity, even after controlling for age, parity and hospital status. Maternal age > 35 years and nulliparity were also significant risk factors for non-immunity.

Programs targeting underimmunised populations for rubella vaccination should focus on overseas-born women, particularly those born in Asia, nulliparous women and also women > 35 years of age.