Every night at 8 p.m., 18-year-old Catherine Msimango takes a pill.
It's the same pill that people with HIV take to fight the virus. Only she doesn't have HIV.
Msimango says the pill gives her power against the virus. She can take it even without her boyfriend knowing.
"It's all about my safety because I don't know what he does when I'm not around," she says. "If he doesn't want to use protection [a condom], I know that I'm safe from the pill."
Msimango lives in the sprawling South African township of Soweto in the heart of the HIV epidemic. South Africa has nearly 7 million people living with HIV, more than any other country in the world. And nearly 1 in 5 adults is infected. HIV rates are lower for adolescents but increase rapidly as teens move into their 20s.
Some AIDS experts now believe that one way to keep rates down is with a daily pill.
The novel prevention technique has proven highly effective in blocking the transmission of HIV in gay men and sex workers. Now it's being tried among sexually active teens.
"I know the experience of HIV," says Msimango, who has lost several close relatives to AIDS. "I don't want to get there."
She says that's why she joined this pilot program last year to offer anti-retroviral drugs to teenagers in Soweto and Cape Town, South Africa.
Studies have shown that taking daily doses of the drugs offers an extremely high level of protection against HIV. If taken correctly and consistently, the pill is nearly 100 percent effective in blocking transmission of the virus. Researchers call the technique pre-exposure prophylaxis, or PrEP.
Those earlier studies were primarily with gay men. The study Msimango is in was launched last year with 150 sexually active teens between the ages of 15 and 19. Linda-Gail Bekker is the deputy director of the Desmond Tutu HIV Centre at the University of Cape Town and one of the lead investigators on this pilot study.
"I think having [a form of HIV prevention] that a young woman can use discreetly and is in her absolute control is something we've been missing throughout this epidemic," Bekker says.
"And I think now for the first time we are able to offer something to young women that is not required at the time of sexual coitus." Expecting teenagers to negotiate the use of condoms in the heat of sexual passion hasn't always been successful, she notes.
Bekker is optimistic about PrEP as a new HIV prevention method. She says it could be revolutionary if it's introduced on a large scale. The study wraps up next year; it's hard to say if the ministry of health will make PrEP available to teens after that.
She and her colleagues are monitoring the teens to make sure there are no ill effects from the drugs. These anti-retroviral drugs have improved dramatically over the past decade; they're far less toxic than they used to be. Doctors, however, continue to be concerned about possible kidney damage from long-term use of these drugs. So far they haven't seen problems.
Indeed, the safety of PrEP isn't the issue. That's now widely accepted. The main purpose of this pilot program is to see how well teens adhere to the daily treatment, says Bekker.
PrEP is about adding more tools to the HIV prevention toolbox, she says. And they're needed. The reality is, she says, prevention messages based primarily on trying to get people to use condoms haven't stopped the spread of HIV.
"Thirty years into this epidemic, it's clear that condoms are not the solution for everyone," she says.
Bekker calls the PrEP pills a "chemical condom."
Globally, new HIV infections continue at an alarmingly high rate. A new study from The Lancet says HIV infections worldwide peaked in 2005 at nearly 5 million and now are reported to have plateaued at 2.5 million a year.
Sabelo Sekhukhuni, one of the counselors helping to run the PrEP program in Soweto, says the pressure on teenage girls in impoverished parts of South Africa to have sex is huge.
"When you look at an informal settlement where there isn't electricity, sex is a sport. Sex is an activity to keep themselves busy," he says.
Sex also at times is used as currency. Girls who have no money can be offered cellphones or new clothes by older men who want to sleep with them.
Just behind the hospital where the PrEP study is being conducted, says Sekhukhuni, there's a settlement of shacks where the people don't have electricity.
"So there's this one guy who owns a tavern and he has a generator, and he makes people pay 5 rand to charge their cellphones," he says.
But he offers to charge girls' phones for free in exchange for sex. And some of the girls accept his offer.
In these types of relationships, the tavern owner holds a lot of power in the community. He's got the generator. The man may be HIV positive, and Sekhukhuni says he may refuse to use condoms.
"The same girls that sleep with this guy, they'll go back and sleep with their peers of their same age group," he says. "Meaning [HIV] is still going to spread some more."
But Sekhukhuni says this is the beauty of PrEP. In these complex, messy, real-world sexual networks, PrEP may be able to protect these teenagers from the lifelong burden of HIV.