40 ) % ( re30 n t r a p s tce20 f n i y t i l i abb10 o r P 0 Never condoms 30% condom use Not if low VL Condom use Always condoms 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 condoms. Error- bars show the 95% uncertainty interval, as described in the text. It is assumed that the partnership is maintained over the entire course of ﬁrst-line treatment and that viral load is related to transmission rate, as Fraser et al20 have described. 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% (0.2e8% 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% (0e7% in uncertainty analysis) (ﬁgure 2). 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 trans- mission (ﬁgure 3A). Patients monitored frequently can be quickly switched to new regimens if ﬁrst-line treatment failsdthat 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 moni- toring, many men with suppressed viral loads would use condoms since they have not had a recent viral load measure- ment. With monitoring every 18 months, condoms would be used in w70% of sex acts, but with monitoring every 3 months, condoms are used in only 9%. 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 (ﬁgure 3B). 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 Sex Transm Infect 2011;87:17e21. doi:10.1136/sti.2010.042622 Epidemiology 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 is maintained over the entire course of ﬁrst-line treatment and that viral load is related to transmission rate, as Fraser et al20 have described. 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 3e12 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. DISCUSSION The debate about the EKAF statement on HIV transmission from patients receiving cART13 and earlier modelling work14 did not consider the way in which deciding not to use condoms might be conditional on the last viral load measurement. There was also little focus on estimating the rate of HIV transmission across the population or exploring how it is inﬂuenced by patterns of patient monitoring. In this paper we have shown that basing the decision to use condoms on viral load provides substantially better protection to partners than incomplete condom use, provided that the measurement is within the past 3e6 months. Compared with always using condoms, the viral-load-dependent strategy allows slightly more HIV transmission (2e3% vs 1%). However, as condoms are needed much less of the time (10% vs 100%), adherence to this strategy may be better. Our results also highlight the importance of monitoring patients more often and minimising losses to follow-up. 19