Girls twice as likely as boys to get HIV from their mothers in the womb

Girls are twice as likely as boys to be infected with HIV in their mothers’ wombs, according to a European study published in the January 23^rd edition of AIDS. Investigators believe that this may be because of genetic differences between the sexes, as well as different immunological responses, and could explain why infant girls have consistently lower viral loads than boys.

The European Collaborative study is a prospective cohort involving infants infected with HIV by mother-to-child transmission. A total of 3231 mother and child pairs enrolled in the study between 1986 and 2003 were analysed by the investigators. The overall mother-to-child transmission rate was 10.6%, declining from 14.4% before antiretroviral prophylaxis was used to prevent mother-to-child transmission, and falling to a little over 2% in the last year of the analysis, when 92% of women took anti-HIV treatment (89% HAART), and 58% had an elective caesarean delivery.

In total, 350 infants were infected with HIV – 170 boys (48.6%) and 180 girls (51.4%). In univariate analysis, the association between sex and mother-to-child transmission was of only borderline significance (p=0.052), but when the investigators adjusted for antenatal HIV therapy, maternal CD4 cell count, and elective caesarean, they found that girls had a 1.5 times higher risk of infection than boys (p=0.029).

The investigators then explored the timing of mother-to-child transmission, and found that the sex effect was limited to the infants of mothers who had had an elective caesarean. Of all the infants born using a caesarean, 6.23% of girls and 2.8% of boys were infected with HIV (p=0.0094). Adjusting for the use of antenatal antiretroviral therapy, and the year of birth, girls were twice as likely as boys to be infected with HIV (adjusted odds ratio 2.1, p=0.017). However, the sex difference disappeared when the investigators controlled for the use of HAART by the mother during pregnancy (although the investigators caution that this may be due to the small number of infections recorded).

"On the crude assumption that infections among infants delivered by [caesarean section] without labour and before membrane rupture are a proxy for intrauterine transmission, our findings suggest that transmission before 36 – 38 weeks…may be more common in girls than boys," conclude the investigators.

The investigators also speculate that the difference in transmission risk between girls and boys could "reflect underlying genetic or immunological differences between the sexes". They suggest that their findings could explain viral load and CD4 cell differences between infant boys and girls, as boys and girls possibly acquired HIV from their mothers at different times.

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