Transmission from mother to child

The link between viral load and mother-to-child transmission is very well established. In the UK, the transmission risk from a pregnant woman on antiretroviral treatment with an undetectable viral load to her child is 0.1% (1 in 1000). Although the risk is greatest for those pregnant women with high viral loads, transmission can occur even when maternal viral loads are very low or undetectable. Data from the UK and Ireland¹ have found that transmission is possible, although extremely unlikely, when the maternal viral load is below 50 copies/ml.

Between 2000 and 2006, 5930 infants were born to HIV-positive mothers. Of the 2309 mothers with an undetectable viral load at, or near, the time of delivery, three mother-to-child transmission events were reported, resulting in a transmission rate of 0.1%. This compares with a transmission rate of 0.8% for women who received antiretrovirals for at least the last 14 days of pregnancy, regardless of type of antiretroviral treatment or prophylaxis or mode of delivery. Not receiving antiretrovirals increased the risk of transmission nine-fold.

In Africa, providing antiretroviral treatment during pregnancy and breastfeeding (which is uncommon in the UK) resulted in a mother-to-child transmission rate of less than 1% in a large randomised comparison of two triple combinations in women with CD4 counts above 200 cells/mm³.²

The investigators in the Mma Bana trial randomised 560 HIV-positive pregnant women with CD4 cell counts above 200 cells/mm³ to start one of two antiretroviral regimens between weeks 26 and 34 of pregnancy. This treatment was continued until weaning six months after giving birth. The women either received Trizivir (abacavir/AZT/3TC), which is not a recommended therapy in the UK, or Kaletra/Combivir (lopinavir/r/AZT/3TC).

Also included in the study were 170 women with a CD4 cell count below 200 cells/mm³. In accordance with treatment guidelines at that time, to protect their own health they started a combination of anti-HIV drugs consisting of nevirapine with 3TC and AZT between weeks 18 and 34 of pregnancy. All mothers also received supplemental AZT (zidovudine) during labour.

After delivery, infants received single-dose nevirapine and then AZT (zidovudine) for one month. The cumulative rates of mother-to-child transmission of HIV were equally low in all three groups of women (2% in the triple NRTI arm vs below 0.4% in the Kaletra group and 0.6% in the nevirapine group) - the differences were not statistically significant. Only two transmissions occurred during the breastfeeding period, in one case from a mother who had viral load below 50 copies/ml at months one and three and no evidence of adherence problems, yielding a transmission rate during breastfeeding of 0.4% on the Trizivir and nevirapine regimens and zero on the Kaletra-based regimen.

Other studies reported at the same time observed higher rates of infection,^3,4 but used regimens less comparable with ones used in the UK and also reported more mixed (breast and bottle) feeding, which is known to increase HIV transmission, as opposed to exclusive breastfeeding.