We found that men receiving treatment pose a substantial risk of HIV transmission (22% (9–37% in uncertainty analysis)) to their partners if they do not use condoms (), and the relationship between viral load and transmission is as described by Fraser et al
This risk is generated in three ways: (i) treatment is not sufficient to suppress viral load, so transmission can occur even if the regimen is quickly changed; (ii) the level of virus can rebound quickly and reach high levels before detection and change of regimen; (iii) even with suppressed viral loads, the risk of transmission is not zero, so that over the many sex acts during treatment, the cumulative chance of transmission becomes non-negligible.
Figure 2 Probability of transmission to partner during first-line treatment, if condoms are never used; condoms are used 30% of the time; (iii) condoms are used unless last viral load (VL) measurement in the past 6 months was undetectable; or always using (more ...)
Using condoms 30% of the time reduces the chance of transmission but only marginally (to 17% (7–29% in uncertainty analysis)), since there remains a substantial numbers of unprotected sex acts overall. In contrast, men who always use condoms unless their viral load was undetectable at the last measurement in the past 6 months are much less likely to transmit HIV to their partner (chance of transmission: 3% (0.2–8% in uncertainty analyses)). This is because the risk generated in the first two ways has been largely removed. Men following this strategy, on average, use condoms 10% of the time they are receiving treatment, while reducing the risk of transmission to 3%. The risk with this strategy is nevertheless greater than always using condoms, in which the chance is 1% (0–7% in uncertainty analysis) (). The risk is not zero, because condom efficacy is not perfect.
Using the alternative (‘Wilson et al’) assumption about the relationship between plasma viral load and transmissibility, the overall chance of infection if condoms are not used is higher (47% vs 22%), but the same key result is reproduced: men who use condoms unless their viral load was undetectable in the past 6 months are much less likely to transmit HIV to their partner than if condoms are used intermittently (online figure S3).
The frequency with which patients receiving treatment are monitored is a key determinant of the chance of HIV transmission (). Patients monitored frequently can be quickly switched to new regimens if first-line treatment fails—that is, before viral load increases and they have exposed their partner to an increased risk of transmission for a long period. The second effect of increased frequency of monitoring is the reduction in overall condom use. This is because with less frequent monitoring, many men with suppressed viral loads would use condoms since they have not had a recent viral load measurement. With monitoring every 18 months, condoms would be used in ~70% of sex acts, but with monitoring every 3 months, condoms are used in only 9%.
Figure 3 The influence of (A) monitoring frequency and (B) loss to follow-up on the probability of HIV transmission, assuming condoms are used unless last viral load measurement in the past 6 months was undetectable. It is assumed that the partnership (more ...)
The benefit of patient monitoring is influenced by the proportion of patients lost to follow-up. The probability of infecting a partner increases from 2% when none of the patients are lost to follow-up, to 5% when 20% do not return for care (). This is because increases in viral load go undetected for longer, exposing partners to a higher risk of infection.
Finally, we examined three ways in which condom use could be based on viral load measurements (online figure S5). The decision not to use condoms could be based on an undetectable viral load in the past 3 months, the past 6 months or the last measurement ever. With the decision based on a measurement in the past 3 months, there is reduced transmission compared with a decision based on the past 6 months, provided that patients are monitored at least every 3–12 months. However, with the decision based only on the last measurement regardless of time, the chance of transmission is higher, especially if monitoring intervals are longer.