A study by the Swiss Federal AIDS Commission on patients who were treated with highly active antiretroviral therapy (HAART) concluded in 2009 that individuals with a stable, low viral load for at least six months were extremely unlikely to transmit HIV. In response, the Swiss HIV Cohort Study sought to determine how consistently viral load remains below detectable levels.
The study concludes that when several successive viral loads are less than 50 copies/mL it remains reliably undetectable approximately 94% of the time with a cut-off of 50 copies/mL, and approximately 99% reliable with a cut-off of 1000 copies/mL.
The most significant factor in maintaining a reliably undetectable viral load is consistent compliance with a patient's HAART regimen. Also affecting reliability was the patient's past drug therapy, i.e., the type of HAART and the first antiretroviral therapy the patient received. Patients who started with HAART had a higher rate of reliability than those who started on NRTI mono- or bi- therapy (the first class of antiretroviral drugs developed) in the 1990s.