One of the main weapons to prevent mother-to-child transmission of the AIDS virus during birth is the drug nevirapine. But when nevirapine is used alone just once, HIV starts becoming resistant to it. Research in Botswana shows that the resistance is not long lasting and that this affordable drug does not have to be abandoned forever by infected mothers who have already taken it. VOA's David McAlary reports.

International medical guidelines call for pregnant women with advanced HIV to get a combination of AIDS drugs includingnevirapineto prevent passing their infection on to their newborns during delivery. But in poor countries, combinations have been expensive and nevirapine has often been used alone, since studies have shown that a singledosecan cut the transmission rate in half.

The problem is that HIV resistance builds against it quickly when used alone just once because other drugs are not present to kill the virus particles that survive nevirapine. This renders the drug less effective in later combinations for treating women after their baby is born.

But the new study from Botswana shows that nevirapine can make a comeback for these women if they wait until the resistance subsides.

"The further out you get from that exposure to single dose nevirapine, the less detectable nevirapine resistance is," said Shahin Lockman of the Harvard School of Public Health in Boston.

She says waiting period for women who get the single dose of nevirapine at delivery can be as short as six months.

"If they started nevirapine-based treatment six or more months after nevirapine exposure, their treatment response was just as good, and really quite high, compared to women who did not have the single dose of nevirapine," she added. "However, the women who started nevirapine-based treatment within six months of that nevirapine exposure were much more likely to experience treatment failure."

The study published in the New England Journal of Medicine shows that waiting at least six months means that HIV-positive women are 70 percent more likely to benefit from nevirapine-based drug combinations again than women who get them sooner.

An official with the U.S. government health agency that helped fund the study calls it very important.

Lynne Mofenson is chief of research on child, adolescent, and maternal AIDS at the U.S. National Institute of Child Health and Human Development. She says the finding supports a World Health Organization (WHO) recommendation restricting a single dose of nevirapine only to pregnant HIV-infected women who are healthy enough to wait six months after childbirth for more nevirapine-based therapy. Otherwise, they should get other drugs during labor.

"It shows the importance of screening women for treatment while they are pregnant and putting them on appropriate therapy while they are pregnant to avoid having to start them too soon after they received preventive therapy," she explained.

Shahin Lockman in Boston says the problem of nevirapine resistance should diminish now that more and more people are receiving combinations of AIDS drugs under expanded U.S. and international programs to deliver them to Africa and other regions hard hit by the virus.