There’s an emerging consensus among AIDS experts that treating HIV earlier reduces mortality, slows the progression of AIDS and may even reduce transmission of the virus. But just how early that treatment should start depends on who you ask.

At the International AIDS Conference in Vienna today, the World Health Organization released new guidelines saying that antiretroviral therapy should be initiated when a patient’s CD4 cell count drops below 350 cells per microliter. The previous WHO standard, adopted in 2006, advised starting therapy when CD4 levels dropped below 200 cells per microliter. Earlier treatment, the WHO says, could reduce AIDS-related deaths by about 20% over the next five years.

The new guidelines, if adopted, would expand the number of people needing treatment to 15 million worldwide. The WHO estimates this increase will push costs for HIV treatment up to $9 billion in 2010. But the added expenditure could be offset by a decrease in hospital costs, since delaying treatment means patients will eventually require more expensive drugs as they become sicker.

Gottfried Hirnschall, the director of the WHO’s HIV/AIDS program, also cites the prevention benefits of early antiretroviral therapy. “Because treatment reduces the level of virus in the body, it means HIV-positive people are less likely to pass the virus on to their partners,” he said in a press release .

The WHO also highlighted a 25% jump in the number of people being treated for HIV in 2009, the largest increase in a single year. It’s estimated that 5.2 million people are currently being treated for HIV.

Meanwhile, the International AIDS Society advocates starting antiretrovirals even earlier, at a CD4 count below 500 cells per microliter. Their guidelines were presented in Vienna on Sunday and appear in this week’s Journal of the American Medical Association. A New England Journal of Medicine study of more than 17,000 patients found that initiating antiretroviral treatment at or above 500 cells per microliter decreased the relative risk of death by 94%. By comparison, starting it between 350 and 500 cells per microliter decreased the relative risk of death by 69%, the researchers found.

Image adapted from Nature Reviews Genetics 5, 52-61 (2004).