The advice will have the most impact in Africa, where nearly 70 per cent of people with HIV live. Many rich countries already advocate early treatment. WHO's new guidelines were released Sunday at the International AIDS Society meeting in Kuala Lumpur, Malaysia.
About 34 million people worldwide have HIV, the virus that causes AIDS. HIV attacks key infection-fighting cells of the immune system known as T-cells. When that count drops to 200, people are considered to have AIDS. In the past, WHO recommended countries start treating people with HIV when their T-cell count fell to 350; a normal count is between 500 and 1,600.
The new recommendations say to treat earlier, when the T-cell count hits 500.
In addition to children younger than 5, WHO says several other groups should also get AIDS drugs as soon as they're diagnosed with HIV: pregnant and breast-feeding women, people whose partners are uninfected and those who also have tuberculosis or hepatitis B.
The guidelines mean an additional 9 million people in developing countries will now be eligible for treatment. At the moment, only about 60 per cent of people who need the life-saving drugs are getting them.
"WHO has recognized that time is the most important commodity when it comes to battling the HIV epidemic," said Sharonann Lynch, HIV policy adviser at Doctors Without Borders, which contributed to the new guidelines.
She said that while the costs for rolling out this treatment might be expensive, the strategy would ultimately result in fewer HIV infections and deaths in the future.
"It's pay now or pay later," she said.
The guidelines also mean the total global spending on AIDS — about $23 billion a year — will rise by about 10 per cent, according to Gottfried Hirnschall, director of WHO's HIV department. It's unclear how willing donors will be to pitch in for even more AIDS treatments.
Hirnschall said the cheapest course of the drugs costs $127 per person every year under programs that have negotiated prices for poor countries, but the price can be much higher elsewhere. WHO's recommended treatment is a single pill that combines three powerful drugs taken once daily.
In the U.S., officials recommend that everyone who has HIV should be on treatment but say there is only "moderate" evidence for starting therapy when the immune system is still working normally.
WHO's new guidelines are based largely on recent studies suggesting people with HIV who start treatment before their immune systems weaken live longer. The case of a U.S. baby girl with HIV who was treated aggressively within 30 hours of being born suggests very early treatment could prevent the virus from ever getting a foothold. Earlier this year, doctors announced the little girl from Mississippi was apparently cured after stopping medication for about a year with no signs of infection.
Several studies have also hinted that starting therapy early dramatically cuts the chances an infected person will pass the virus to a sexual partner.
If all countries start treating people with HIV in line with the new recommendations, WHO estimates 3 million lives could be saved and 3.5 million new infections could be avoided in the next decade.
But convincing people to take a lifelong regimen of drugs that come with side effects including liver problems and severe skin reactions, will be challenging.
"These drugs are not like sweeties," said Dr. Sarah Fidler, an HIV expert at Imperial College London who is leading a trial in Africa studying issues including the effectiveness of immediate treatment for people with HIV. She had no role in the WHO guidelines.
Studies in Africa have shown varying compliance rates from 50 per cent to more than 90 per cent, similar to elsewhere in the world. If patients aren't taking their medicines at least 70 per cent of the time, that could also lead to drug resistance.
Fidler said that while the WHO guidelines are a step in the right direction, implementing them would not be easy.
"For people struggling with other issues like poverty, taking pills for a disease that isn't making them sick yet might not seem like the most important thing in the world," she said. "This is not going to be as simple as just giving drugs to everybody."