For years scientists have debated, heatedly, what portion of HIV infections are transmitted from people who just caught the bug themselves. Though estimates have ranged from just two to three percent to as high as 75 percent, the commonly held assumption is the majority of HIV cases originate from newly infected people. A new study challenges this with a new suggestion that people in the acute phase of HIV may not be as highly infectious as previously believed.

“Our new estimate for the acute phase hazard is nine times less than the currently most frequently used estimate,” noted the authors, who further suggested antiretroviral therapy used as a prevention strategy is sound based on these results.

Three Stages

Without treatment, HIV infection advances in stages, growing worse over time. A few weeks after becoming infected, an HIV-positive person enters a months-long period known as the acute phase of infection, when levels of virus in their bloodstream spike. Following this, the disease advances to chronic HIV infection, during which time the virus continues to multiply in the body but at very low levels. People with chronic HIV infection may not have any symptoms, but they can still spread HIV to others. Without treatment with HIV medicines, chronic HIV infection usually advances to acquired immunodeficiency syndrome or AIDS in 10 to 12 years.

To understand exactly how infectious people are at the acute phase, researchers commonly have used data from the Rakai study, an investigation of HIV transmission within 235 Ugandan couples. Quite simply this is the only study to ever measure infectivity directly. Over the years different analyses of this data have come to different conclusions, but all indicate infectivity is higher during the early stages of HIV than during the chronic stage. In the new study, the researchers used mathematical models to reanalyze this same dataset. Unlike their predecessors, they discovered some previously unseen confounding factors.

Crunching the numbers anew, they discovered acute infectivity to be five-fold higher than chronic stage infectivity. However, their estimate is substantially lower — one-tenth the amount of a commonly agreed estimate for acute infectivity. This revised estimate also happens to be consistent with assessments derived from viral load calculations. Most likely this is news public health officials would rather not spread. Some people could misinterpret the study and believe it means they need not worry about getting tested and learning their diagnosis as soon as possible. However, the results offer mostly hopeful news since they support many of the public health measures already in place.

“Bellan and colleagues’ study has shaken our faith in a result taken for granted for a decade,” Dr. Laith J. Abu-Raddad, Infectious Disease Epidemiology Group, Weill Cornell Medical College in Qatar, wrote in an editorial. However, Abu-Raddad also surmised, “These findings affirm the role that treatment as prevention can play in controlling the epidemic.”

Source: Bellan SE, Dushoff J, Galvani AP, Meyers LA. Reassessment of HIV-1 Acute Phase Infectivity: Accounting for Heterogeneity and Study Design with Simulated Cohorts. PLoS Med. 2015.