Liz Szabo

USA TODAY

Although a daily pill can prevent HIV infection, very few people actually take it.

About 1.2 million Americans are at high risk for HIV and could benefit from taking the pill, sold under the brand name Truvada, in a strategy that doctors call “pre-exposure prophylaxis,” or PrEP, according to a new report from the Centers for Disease Control and Prevention.

Studies in men who have sex with men show that taking PrEP reduces the risk of contracting HIV by 92%. Studies in injection drug users have found that PrEP reduced the risk of infection by more than 70%.

Yet only about 21,000 people – less than 1% of those who could benefit – are taking PrEP, the CDC’s Jonathan Mermin said.

Although the Food and Drug Administration approved Truvada in 2012, one-third of primary care doctors have never heard of PrEP, according to a national survey that has not yet been published.

"We have this sea change in HIV prevention, but it is one of the best-kept secrets in medicine," said J.D. Davids, managing editor at TheBody.com, an online resource on HIV and AIDS.

Actor Charlie Sheen's recent HIV disclosure could help spread the word.

In an interview on The Dr. Oz Show last week, Sheen's former girlfriend, Amanda Bruce, said she took medication to prevent becoming infected with HIV.

Sheen has said that medication reduced his HIV levels to an undetectable level. Although he disclosed that he has not always used condoms, he said that his sexual partners knew about his diagnosis and were under the care of his doctor.

Treating people with HIV with effective drugs can reduce their viral loads to levels that are "undetectable," meaning that the virus can't be found with standard tests. Suppressing the AIDS virus in this way helps patients to stay healthier. But it also makes it almost impossible for them to spread the virus to others. Doctors describe this phenomenon as "treatment as prevention."

Only about 30% of people with HIV have their virus well-controlled, Mermin said.

Charlie Sheen luckier than most with HIV thanks to 'undetectable' virus

Sheen's story illustrates how scientific advantages have changed the nature of HIV prevention.

For 30 years, doctors' best advice for preventing HIV transmission was to use condoms or to practice monogamy with a faithful, HIV-negative partner. That advice has failed to halt HIV transmission.

Today, however, HIV infections are falling, and it's possible that the new medications are part of the reason. The number of new HIV diagnoses declined from 43,862 in 2010 to 39,877 in 2014, according to the CDC.

"PrEP has the potential to dramatically reduce new HIV infections," Mermin said. "However, PrEP only works if patients know about it, have access to it and take it as prescribed."

Yet doctors such as Paul Sax, clinical director of infectious disease clinic at Brigham and Women’s Hospital in Boston, say they've seen an increase of awareness of PrEP in just the past six to 12 months.

One study found that about 15% of gay men in San Francisco are taking PrEP, according to the CDC.

In New York, prescriptions for PrEP among people with Medicaid insurance jumped dramatically after the state launched a campaign to tell people about the medication. Prescriptions climbed from 303 from July 2013 to June 2014 to 1,330 from July 2014 to June 2015.

Doctors say there are practical barriers to getting PrEP to everyone who could benefit.

Although relatively few patients have serious side effects from PrEP, the pills can cause kidney and bone damage, Sax said. Doctors need to perform blood tests before prescribing Truvada, to check patients' kidney function. Patients taking PrEP need to see doctors every three months to make sure their kidneys are functioning properly and that the patients haven't become infected with HIV.

Sax says he doesn't recommend PrEP for people at relatively low risk of contracting the virus.

Common questions about HIV answered

Truvada, made by Gilead Sciences Inc., costs about $10,000 a year, according to the CDC.

Many insurance plans cover PrEP, and uninsured patients can get Truvada for free through Gilead's patient assistance program, said Carlos del Rio, professor of global health and medicine at Emory University and co-director of the Emory Center for AIDS Research in Atlanta.

Although state AIDS Drug Assistance Programs pay for drugs to treat HIV, they don't pay for PrEP, Sax said.

Some state Medicaid programs don't pay for PrEP either, del Rio said.

Although PrEP is an important strategy for preventing HIV, del Rio said local health departments aren't allowed to pay for PrEP with money earmarked for prevention from the CDC. The federal Ryan White Program, which pays for HIV and AIDS treatment, also doesn't pay for PrEP, he said.

"We have a broken health care system that makes implementation of this kind of thing very difficult," del Rio said.

Today, taking PrEP involves taking medication every day, much as women take a birth control pill.

Preliminary results from a study presented earlier this year, however, suggest that PrEP is also effective if taken "on demand," before and after sexual encounters. This strategy involved taking two pills two to 24 hours before intercourse, a third pill 24 hours later and a fourth pill 48 later.

Taking PrEP "on demand" in this way reduced the risk of contracting HIV by 86%, compared to a placebo, according to the study, presented at the annual meeting of the Conference on Retroviruses and Opportunistic Infections. The study, which has not yet been published in peer-reviewed journal, involved more than 400 gay or bisexual men who have sex frequently. Mermin said it's not clear if taking PrEP on demand will be effective in other groups.

"We have everything we need to really tackle HIV transmission," Davids said. "It really should be shouted from the rooftops."