An official with the World Health Organization (WHO) said Thursday that the HIV drug resistance needs to be observed, although it is not a major problem right now.

"We have to be vigilant. We have to monitor the evolution and do some surveillance of drug resistance," Gottfried Hirnschall, director of the WHO's Department of HIV/AIDS, told Xinhua during the six-day International AIDS Conference starting on July 22.

As the antiretroviral therapy (ART) for HIV infection has saved millions of lives since it was introduced and its coverage continues to grow, some degree of emergence and transmission of HIV drug resistance is inevitable.

According to Hirnschall, at the end of 2011, more than 8 million people were receiving antiretroviral therapy in low- and middle-income countries, a dramatic 26-fold increase from December 2003. The WHO initiated global surveillance of HIV drug resistance as countries scaled up access to antiretroviral therapy.

HIV drug resistance can be categorized as transmitted resistance, which occurs when previously uninfected individuals are infected with a drug-resistant virus, and acquired resistance that turns up when resistance mutations emerge because of drug selective pressure in individuals receiving antiretroviral therapy.

The WHO official said published studies suggest that the prevalence of transmitted drug resistance in low- and middle-income countries increased between 2003 and 2009 to levels of about 6 percent, and the overall prevalence of acquired HIV drug resistance was 5 percent, ranging from 4.8 percent in 2007 to 6.8 percent in 2010.

Data also suggest that 10 percent to 17 percent of the people being treated in high-income countries like Australia, Japan and the United States, are infected with virus resistant to at least one antiretroviral drug.

"The major difference is that in high-income countries, treatment with ARVs (antiretroviral drugs) starts much earlier and it starts with a single drug," Hirnschall said. "When the first drug came out, they were given as a monotherapy. And that led very quickly to drug resistance."

Fortunately, WHO surveys indicate that, if drug resistance is detected in time and infectors are switched to second-line regimens soon after virological failure, standard second-line treatment combinations are likely to be effective for the majority of patients failing first-line therapy.

Hirnschall said the drug resistance is categorized into three levels. Between zero to five percent of prevalence of drug resistance was considered low; between 5-15 percent considered moderate; and above 15 percent considered high.

"So right now, it is not a major problem in low- and middle-income countries," said Hirnschall. "But countries with moderate drug resistance need to very carefully monitor to see whether it will go up or stay the same or what the trend is."

Hirnschall said lack of surveillance data over time substantially limits the ability to assess trends in low- and middle-income countries.

"It's too early to say at what level and when drug resistance will plateau in these countries," he said. "We have to do everything to make their programs most effective, keeping their treatment and providing regular drug supply to them."