This Chart Shows Why HIV Is Still Spreading in the U.S.

HIV-1 budding (in green) from a cultured lymphocyte.

Source: CDC

Each year in the U.S., about 50,000 people contract HIV, the virus that causes AIDS. That rate hasn’t changed much in the last decade, even though anti-retroviral therapy can effectively control the virus and dramatically lower the risk that people with HIV will transmit it.

If you want to understand why HIV continues to spread in the U.S., take a look at the chart below. It’s based on new estimates, published today in the journal JAMA Internal Medicine, of what stage of care people with HIV are in when they’re most likely to transmit the virus to others.

The chart describes what epidemiologists call the HIV “continuum of care.” That’s a series of steps between when people contract the virus and when, with proper medical care, the virus in their bodies is suppressed. It includes getting diagnosed, getting into care, getting a prescription for anti-retroviral therapy, and ultimately lowering the amount of virus in the body to a level that allows people to live longer, healthier lives and reduces the risk that they’ll transmit HIV to others.

Almost half of the HIV-positive population in the U.S. has been diagnosed but isn’t getting medical care. This group accounts for the greatest number of new transmissions—more than 60 percent, by researchers’ estimates. Most new transmissions involve men who have sex with men. The estimates are based on data on the 2009 HIV-positive population in the U.S. of 1.15 million Americans. The Centers for Disease Control and Prevention’s latest estimates put the number at 1.2 million.

About 37 percent of Americans with HIV are getting care, and together they account for only 8.5 percent of new transmissions, according to the JAMA paper’s estimates. Reducing the rate of new infections depends on moving more people along the continuum.

“Focusing national HIV prevention efforts on increasing the percentage of HIV-infected persons who are diagnosed and retained in medical care that leads to immediate prescription of [antiretroviral therapy] will contribute substantially to reducing HIV transmission in the United States,” the researchers, from the CDC and Emory University, write. Anti-retroviral therapy, or ART, can effectively prevent HIV-positive people from developing AIDS, which attacks the immune system and leaves the body vulnerable to other infections.

Just getting people into medical care, without starting anti-retroviral therapy, was linked to reduced risk of transmission. The authors speculate that the other services people get, such as counseling, might influence behavior.

While it’s not clear why so many HIV-positive Americans aren’t getting proper treatment, some combination of access to health care and shame is likely at work. “There’s still a lot of stigma associated with HIV. That’s another reason why people don’t get treatment,” Carl Schmid, deputy executive director of the AIDS Institute, told me in December.

Between 2008 and 2010, the number of new infections among men who have sex with men under age 24 increased by 22 percent. HIV affects people from all backgrounds in the U.S., but some populations are disproportionately affected, particularly blacks, Hispanics and men who have sex with men. Increasing rates of new infections, particularly among young black men who have sex with other men, have raised concerns in recent years.

Globally, about 35 million people are infected with HIV, according to the World Health Organization. About 70 percent of the HIV-positive population lives in sub-Saharan Africa, which accounts for the same share of new transmissions. The virus was linked to 1.5 million deaths in 2013.