The high rates of nevirapine resistance seen after single dosing probably result from its very long half-life in the blood. During this period, the virus continues replicating in the presence of drug. Studies have reported that women may still have detectable plasma concentrations of nevirapine up to three weeks after taking a single 200mg nevirapine dose (Cressey 2005; Muro 2005).

A few studies suggest that giving a short course of nucleoside reverse transcriptase inhibitors (NRTIs) after birth to either the mother or infant exposed could cover nevirapines 'tail' and prevent the virus from being exposed to potent nevirapine monotherapy.

A South African study of 226 mother / infant pairs found that adding four to seven days of AZT and 3TC as twice-daily Combivir after birth to mothers who received single-dose nevirapine during labour reduced the risk that they will develop resistance to nevirapine sixfold, and may preserve their future treatment options. No resistance to AZT or 3TC was detected in the study.

Adding the NRTIs also reduced maternal viral loads significantly, possibly explaining the dramatic reductions in the incidence of HIV transmission and of resistance in the few infected infants from 66 to 0% by addition of AZT and 3TC (Gray 2005b; McIntyre 2005).

The most recent data from the French Ditrame Plus study confirm these findings, showing reduced HIV transmission and nevirapine resistance after adding AZT and 3TC to the mother's single-dose nevirapine for three days and treating the infant with the two drugs for seven days on top of its nevirapine dose (ANRS 1201/1202 DITRAME PLUS Study Group 2005).

Another way to keep a mothers treatment options open would be simply to skip the maternal nevirapine dose, and give single-dose nevirapine to the infant. This idea has been explored in a few studies, including the Botswanan Mashi study and the NVAZ trial in Malawi (Shapiro 2005; Taha 2003).

In general, these have shown that omitting single-dose nevirapine treatment of the mother brings about slightly less impressive reductions in HIV transmission than the use of single-dose nevirapine in mothers and infants. More trials are needed to address this issue directly and to provide clearer indications of the relative risks and benefits of this treatment strategy.