Viral load in semen, vaginal fluid and rectal secretions

One of the most important possible vulnerabilities of Universal Test-and-Treat as an effective method of controlling HIV is that clinic viral load tests are always of viral load in blood plasma, whereas for sexual transmission it is the viral load in semen and in vaginal and rectal secretions that are of importance.

While researchers are in general agreement that viral load in blood and in other body fluids are tightly correlated, and most people on successful treatment will have a reduced viral load in both their blood and sexual fluids, the correlation is not perfect. This means that a certain proportion of people who may have undetectable viral loads according to blood tests may still be infectious.

This may not matter on a population level, where the aim is to bring down average infectiousness and the bulk number of infections transmitted, but may have profound implications for individuals basing sexual-risk decisions on their viral load or that of their partner.

There may also be different degrees of correlation between blood plasma viral load (BPVL) and genital viral load. It has been found, for instance, that the correlation between BPVL and vaginal secretions is weaker than it is between blood and semen. The same may also apply to rectal secretions, though there is very little research on this.

The other significant factor in genital viral load is that there is abundant evidence that inflammatory STIs such as gonorrhoea and herpes cause increases in genital viral load in untreated people, and some evidence this extends to people on HIV treatment.