How do antiretrovirals reduce transmission?

Antiretroviral therapy can reduce the risk of mother-to-child transmission of HIV. This was first demonstrated with AZT (zidovudine, Retrovir)¹ and has since been done using other antiretrovirals (ARVs).

There is a close relationship between viral load and risk of transmission. A woman with a high viral load appears more likely to transmit HIV to her baby, 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-child transmission by reducing levels of virus in cervical and vaginal fluid.

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

However, there is also evidence that infection can occur as early as eight weeks, and that up to a third of infections may occur during the second trimester (months three to six of the 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.

Treatment is also taken by the new-born baby during the first few weeks of life. It could be acting like post-exposure prophylaxis.