How do antiretrovirals reduce transmission?

A mother with high viral load appears more likely to transmit HIV, although a small proportion of women with very low or even undetectable viral load still transmit HIV to their infants. One way in which ARVs may work is by lowering maternal viral load.

ARVs may also reduce the risk of mother–to–baby transmission by reducing levels of virus in cervical and vaginal fluid.

It is estimated that up to 70% of mother–to–baby infections occur during the last weeks of pregnancy and during delivery.

However, there is also evidence that infection can occur as early as 8 weeks, and that up to a third of infections may occur during the second trimester (three to six months pregnancy). Current clinical practice is to avoid use of drugs during the first trimester unless the mother is already on treatment. In these circumstances it is feared that stopping and starting treatment, with its attendant risk of a rise in viral load, may actually increase the risk of transmission.

It is also likely that the treatment received by the new–born child during the first few weeks of life plays a substantial role. It could be acting like post–exposure prophylaxis among healthcare workers who prick themselves on an HIV–exposed needle; the prompt use of ARVs after such accidents has been shown to reduce the risk of the worker becoming infected with HIV by 79%.

Real world impact since 1993

ARV use during pregnancy appears to have had a very significant impact on mother–to–baby transmission in countries where its use has been widely adopted. In France there has been a two–thirds reduction in mother–to–baby transmission since the adoption of a policy of offering maternal AZT treatment in 1994. In the US state of North Carolina researchers found that perinatal transmission declined from 21% of infants born to HIV–positive mothers in 1993 to 6.2% in the first half of 1996. During the same period the proportion of HIV–positive mothers who took AZT during pregnancy rose from 18% to 89%.