Half of all HIV transmissions happen when newly infected people may not know they are carrying the virus, a new Canadian study suggests.

Dr. Mark Wainberg, director of the McGill AIDS Centre, and his team followed 2,500 HIV patients in Montreal over eight years to quantify how newly infected people spread the virus.

Newly infected patients were eight times more likely to transmit the virus than those in the chronic stage of AIDS, when both groups behaved the same way, the researchers report in the April issue of the Journal of Infectious Diseases.

People may not realize their HIV status when they are highly infectious during the first six months, Wainberg said.

"It means we have to reconsider a lot of what we're doing, both on the public education front and on the early intervention front." 

When people are newly infected, HIV rapidly makes copies of itself while the immune system produces antibodies to fend off the virus. Some people may report flu-like symptoms while others do not.

"Early infection accounts for approximately half of onward transmissions in this urban North American study," the study's authors concluded. "Therapy at early stages of disease may prevent onward HIV transmission."

Studies suggest that when people are diagnosed as being HIV positive, they may change high-risk behaviour, such as not using condoms.

"So the more actively we can seek out and find newly infected people for testing and counselling, the better," Wainberg said in a release.

Drugs for prevention

In a journal editorial accompanying the study, Deenan Pillay of University College London and Martin Fisher of Sussex University Hospital said that given the results of this and other studies, it is time to evaluate using antiretroviral therapy, a treatment for HIV, for prevention.

Antiretroviral therapy, though, does not replace the need for behavioural approaches to reduce transmission, the editorial cautions.

The therapy is also expensive, relatively toxic and many regions are not using it to treat infected populations, never mind for prevention, they said.

Clinical trials are underway to test whether the drugs can also be used as part of a prevention strategy among high-risk groups such as sex trade workers.

About 58,000 people are living with HIV in Canada, but about a quarter of them, or almost 14,500, don't know they are infected, the Public Health Agency of Canada said in a report on the estimated number of HIV and AIDS cases in 2005.