Where New HIV Infections Come From: Transmission Along the Care Continuum, 2016

Summary

• People with HIV who take treatment and keep their virus suppressed don't transmit HIV sexually.
• A CDC model of 2016 found that new infections came from people who didn't know they had HIV or whose infection wasn't controlled — about 80% from people undiagnosed or not in care.
• Ending the epidemic means diagnosing HIV early and getting people into sustained care and treatment quickly.

Background

In 2016, about 1.1 million people in the U.S. had HIV, and there were 38,700 new infections. Knowing one's status, behavior change and antiretroviral therapy (ART) all prevent transmission. Most people reach viral suppression — fewer than 200 copies of HIV per mL of blood — within 6 months of starting ART, and can then live long, healthy lives. Four recent studies of couples in which one partner had HIV found no sexual transmission from a partner who was treated and suppressed, across tens of thousands of acts of sex without condoms or preexposure prophylaxis. Today's treatment can be as simple as one pill, with fewer side effects.

Yet many people aren't effectively treated. In 2015, of everyone with HIV, 14.5% hadn't been diagnosed, 37.2% weren't in care, and 48.9% weren't virally suppressed.

The model

CDC updated its Progression and Transmission of HIV (PATH 2.0) model, which follows people with HIV through the stages of care, with sexual and needle-sharing partnerships and viral load determining the chance of transmission. Behavior data came from National HIV Behavioral Surveillance, and prevalence, incidence, care and suppression data from the National HIV Surveillance System. People with suppressed virus were assumed to transmit no HIV.

Results

The overall transmission rate in 2016 was 3.5 new infections per 100 person-years.

People with HIV and HIV transmissions along the care continuum. Figure from the CDC report.

• By group: 73.0% of transmissions came from men who have sex with men (MSM), 9.7% from people who inject drugs, 5.3% from MSM who inject drugs and 12.0% from heterosexuals. Rates were highest among MSM (4.4) and lowest among heterosexual women (1.2).
• By age: the rate was highest at 13–24 (5.1), but people 55 and older — simply because there are more of them living with HIV — generated the most new infections (29.4%).
• Over time: the overall rate fell from 4.5 in 2010 to 3.5 in 2016, as viral suppression rose steadily.

Viral suppression and HIV transmission rates over time. Figure from the CDC report.

What it means

The federal initiative to end the HIV epidemic within 10 years starts with early diagnosis and fast entry into care.

• Diagnose sooner: about 15% of people with HIV were undiagnosed yet caused about 38% of transmissions, and the median time from infection to diagnosis is 3 years. CDC recommends that everyone aged 13–64 be tested at least once and people at high risk at least yearly; community partners can reach people who rarely see a doctor.
• Link and keep people in care: the 23% diagnosed but not in care caused about 43% of transmissions. Barriers include lack of insurance, housing or transportation, stigma, mental health and substance use issues and mistrust of medicine; tailored support services can help.
• Support treatment: the 11% in care but not suppressed caused about 20%. About 80% of people in care are suppressed at their latest visit, but about a third don't stay suppressed for a year.
• Share the message: knowing that suppression prevents sexual transmission matters to many people with HIV and may reduce stigma.

Limits: behavior data were limited and mostly self-reported; no data exist on whether suppression prevents transmission through shared needles (the model assumed it does); the model didn't vary awareness or suppression by age, so rates among young people may be underestimated; it modeled 23,000 people to represent 1.1 million; and it gave no benefit to lower but unsuppressed viral loads.

Sources

Based on Zihao Li, David W. Purcell, Stephanie L. Sansom, Demorah Hayes and H. Irene Hall, "Vital Signs: HIV Transmission Along the Continuum of Care — United States, 2016," MMWR, Centers for Disease Control and Prevention; a work of the United States government in the public domain.