It has now been well established that for someone whose viral load has been undetectable for some months, and who is currently on antiretroviral therapy (ART), then the risk of them transmitting HIV is extremely small – scientists hate to say never, but the risk is negligible. So researchers are trying to get a better understanding of situations in which there is a risk of HIV being passed on. They looked for pairs of people whose HIV appears to be closely related. It’s likely (but not certain) that HIV was passed from one of these people to the other. Out of over 11,000 people, they identified several hundred samples which were closely related.

For around half the samples, the researchers knew both roughly when the person became HIV-positive and when they were diagnosed.

They found that just under half (44%) of transmission pairs included someone who had themselves been HIV positive for less than a year. This confirms several other studies which have found that people with recent HIV infection (who generally don’t know that they have HIV) are responsible for a disproportionate number of HIV transmissions.

Looking at the HIV transmission which happened later on in a person’s infection, the researchers examined 121 cases:

  • 67 involved a person not taking HIV treatment.
  • 18 involved someone who had only recently started taking HIV treatment and who did not yet have an undetectable viral load.
  • 16 involved someone who had started – and then stopped – taking HIV treatment.
  • Data were missing or unclear for the other 20 cases.

This again shows the success of HIV treatment in preventing HIV from being passed on. But it also highlights two periods of vulnerability when transmission can happen: firstly, in the first few months of taking treatment, before it has fully got HIV under control. Secondly, if people take a break from HIV treatment.

The full article can be read here.