If an American becomes infected with HIV, chances are he or she contracted the virus from someone who didn't know they were infected or wasn't getting proper treatment.
That's the message of a new U.S. study, which found that undiagnosed and untreated people with HIV may be responsible for more than nine out of 10 new infections.
The findings "highlight the community-wide prevention benefits of expanding HIV diagnosis and treatment in the United States," a team led by Dr. Jacek Skarbinski, of the U.S. Centres for Disease Control and Prevention, wrote in the report.
Looking at 2009 data, Skarbinski's team said that about 45,000 new cases of HIV were transmitted that year, adding to the total of more than 1.1 million Americans who were already living with HIV, the virus that causes Aids.
Read: How HIV is transmitted
Using national databases, the investigators estimated that more than 18 percent of that total remained undiagnosed, while another 45 percent were aware of their status but were not getting medical care.
Only about one-quarter of HIV-infected Americans had managed to get their viral status under control by using the current standard of care known as antiretroviral therapy, the researchers found. These drugs can lower an HIV patient's viral load to undetectable levels.
The study findings are published in the 23 February online edition of the journal JAMA Internal Medicine.
Untreated patients more likely to infect others
The findings raise concerns about efforts to contain the spread of HIV, given that untreated patients are more likely to infect others than those who are receiving therapy and have been able to suppress the virus in their system, the study authors explained in a journal news release.
Treated patients who have been able to suppress their virus are 94 percent less likely to transmit HIV than those with undiagnosed HIV, the researchers found.
So the vast majority of new infections in the United States – nearly 92 percent – likely occur after contact with people who don't know they carry HIV, or do not receive treatment, according to the CDC team.
In a commentary accompanying the report, Dr. Thomas Giordano, from the DeBakey Veterans Affairs Medical Centre in Houston, wrote that "the study demonstrates that the steps of the cascade that propel HIV transmission in the United States are delayed diagnosis and inadequate retention in care."
However, Giordano added, "What is surprising is the magnitude of the effect of those steps."
Image: Rapid HIV test from Shutterstock