Baby boys less likely than girls to be infected with HIV by their mothers

The rate of mother-to-child transmission of HIV is significantly lower for boys than girls, Italian investigators have found. In a study published in the December 1^st edition of the Journal of Acquired Immune Deficiency Syndromes the researchers from the Italian Register for HIV Infection in Children report that boys had a lower risk of acquiring HIV infection from their mothers than girls, even after they took into account the use of measures to prevent transmission of HIV, including caesarean deliveries and the use of prophylactic use of antiretroviral drugs.

The role of gender in HIV disease progression has been hotly debated. Although some studies have found that gender may be a factor, others have contradicted this. It is also thought that studies suggesting that gender plays a role have not properly controlled for factors such as adherence, behaviour patterns, and lifestyle factors which may differ between men and women.

There are also conflicting data about the importance of gender to the risk of mother-to-child transmission of HIV, and to help provide further information about this “intriguing issue” Italian investigators conducted an analysis of the rate of mother-to-child transmission by gender in a large prospective cohort involving over 4000 infants.

Rates of mother-to-child transmission were examined in two time periods: 1985 to 1995 (before effective antiretroviral therapy was available), and 1996 to 2001. Possible factors of mother-to-child transmission which the investigators considered were infant gender; gestational age at delivery; maternal health; mode of delivery; and the use of antiretroviral prophylaxis (none used vs AZT monotherapy vs combination antiretroviral therapy).

A total of 4151 children were included in the investigators’ analysis. The overall transmission rate was 478 (12%). The rate of transmission differed according to gender and was 10% amongst boys and 13% amongst girls, a statistically significant difference (p = 0.008).

The investigators then looked at transmission rate in the study’s two separate time periods. They found that between 1985 and 1995 the rate of transmission was broadly similar in boys (16%) and girls (18%). However, after 1996 a significant difference in transmission rates emerged: the percentage of boys vertically infected was 3% compared to 6% of girls (p = 0.002).

Further analysis was then performed to see if the mode of delivery or the use of antiretroviral drugs to prevent vertical transmission made any difference to these results. The investigators found that “the protective effect of male gender for mother-to-child transmission persisted independent of type of delivery.” Boys were also found to have a lower risk of infection if no anti-HIV drugs were used to prevent transmission or when AZT was the only drug used. However, no difference between the sexes was seen when mothers were treated with combination antiretroviral therapy. The investigators note that interpretation of this finding was very difficult because of the very low number of infections which occurred.

“We observed a lower proportion of HIV-infected male infants compared with female infants in children born after 1995”, write the investigators.

They speculate about the possible reasons for their findings. Girls, they observe, seem to be more susceptible to human T-cell leukemia/lymphoma virus than boys and that mother-to-child transmission of hepatitis C virus may occur more frequently in girls than boys. However, another explanation could be that the rate of live births may have been lower in boys, as mortality in the womb is higher for male foetuses.