aDepartment of Public Health and Policy, London School of Hygiene and Tropical Medicine, UK
bProgram for Appropriate Technology in Health (PATH), Washington, DC, USA.
Received 19 April, 2005
Accepted 10 May, 2005
In a recent paper in AIDS, Smith et al.  used mathematical modelling to explore the issue of condom migration, or ‘condom replacement’, on the potential impact of microbicide use by female sex workers. Similar to an earlier paper that we published , they developed a static model of the risk of HIV acquisition, and used this to obtain a threshold level of microbicide efficacy and use required to offset condom replacement. Although there are some differences in the mathematics used to formulate the risk of HIV infection and the scenarios considered, in most respects our two analyses yielded similar conclusions. In particular, both concluded that condom migration/replacement is not a substantial concern in populations that have low levels of condom use (as did Karmon et al.  in a similar analysis).
However, we are concerned that only the positive policy conclusions are presented by Smith et al.  with regard to condom replacement among sex workers: ‘For low/moderate efficacy microbicides, the risk of HIV acquisition in FSWs will be reduced – even if complete condom abandonment occurs – if prior condom use was low’.
This conclusion ignores the fact that, with proper counselling and support, sex workers can achieve a high rate of consistent condom use with their paying clients. A recent review of surveys of sex workers in Asia generally found that the median percentage reporting using a condom with their clients in the last sex act was over 75% [4,5]. There is also growing evidence across different settings that, following intervention, sex workers frequently achieve high levels of reported condom use with their clients [6–11], with as many as 96% of male military conscripts reporting using a condom at last commercial sex in northern Thailand, for example . Although there exist many sex worker populations who have low levels of condom use with their clients [5,12–15] (and many more that have not been studied), it is important to acknowledge when considering microbicide introduction scenarios that in some settings sex workers have attained high levels of condom use.
Our analysis suggests that among groups of sex workers with high levels of condom use, migration may be of concern. We found that if sex workers originally used condoms in over 53% of sex acts but abandoned condoms altogether after the introduction of a 50% efficacious microbicide, then the risk of HIV will increase even if microbicides are used in every sex act . If Smith et al.  had considered higher levels of condom use, they would have reached similar conclusions. For example, if Fig. 3 (b) had been extrapolated for higher levels of condom use, then a threshold for a 50% efficacious microbicide can also be seen . From this, it appears that if sex workers had originally used condoms in over 62% of sex acts but abandoned condoms altogether after microbicide introduction, then the risk of HIV will increase even if microbicides are used in every sex act.
Our analysis also showed that if a microbicide of 50% efficacy against HIV and sexually transmitted infections is used in 50% of sex acts not protected by condoms then high-consistency condom users (who use condoms in 90% of sex acts) could only reduce condom use to 86% without increasing the risk of HIV . However, this ‘high-consistency condom-user’ scenario was not discussed in the paper by Smith et al. .
The papers highlight the fact that further research is needed to explore the issue of microbicide introduction to sex workers. If women are able consistently to negotiate condom use with their clients, both models suggest that there may be risks associated with microbicide introduction if it weakens women's ability or resolve to negotiate condom use. At the same time, this result should not be used to withhold or limit the ability of sex workers to access microbicides . Clearly, there is an urgent need for microbicides, to provide additional protection to the many sex workers who are unable to negotiate consistent condom use with their clients [5,12–15], and to the many more who cannot use condoms in their non-commercial relationships (despite achieving high levels of use with clients) [4,5,9,11,17–19]. Instead, such studies raise important programmatic challenges about how best to promote microbicide use in a way that does not undermine consistent condom use, recognizing that there are small margins for error. For, although mathematics can provide a quantification of the risks and benefits, operational and social science research is needed to identify how best to respond.
The views and opinions expressed are those of the authors alone.
Sponsorship: This work is supported by the Global Campaign for Microbicides at PATH, with funding from the US Agency for International Development. P.V. and C.W. are part of the Microbicides Development Programme, which is funded by the UK Department for International Development (DFID). A.F., C.W. and P.V. are members of the DFID-funded HIV/AIDS and STI Knowledge Programme. The UK DFID supports policies, programmes and projects to promote international development.
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