1. ## Higher-than-85% condom use rate in persons with multiple sexual partners is required to control HIV

Dear Editor,

We read with interest the paper by Brown and Peerapatanapokin regarding estimated condom use rate required to control the HIV epidemic in Thailand.[1] Their Asian Epidemic Model predicts that a sustained high condom use rate (85% or more) among sexual workers is required to control the HIV epidemic. If the condom use rate drops to 60%, their model predicts a resurgence of the HIV epidemic.[1] We reached a similar conclusion using a different model developed for a different situation in Taiwan[2], where prevalence of HIV infection is as low as 0.019% (end of 2002) and highly active antiretroviral therapy has been freely provided by the government since 1997.

Sexual contact (96.4%, n=4,390 at the end of 2002) has been the predominant risk factor for acquiring HIV infection in Taiwan.[2] Because HIV prevalence is extremely low, evolution of the HIV epidemic in Taiwan can be approximated by a simple exponential model defined by the equation: dN(t)/dt = R*N(t) - m*N(t), where N(t) is the number of patients with HIV infection at time t, R is the average transmission rate (new cases per prevalent case per year) and m is the risk of mortality (deaths per prevalent case per year). The magnitude of the HIV epidemic will decrease only if R < m.

Using our previously published estimates for R and m values in Taiwan (R=0.184 and m=0.046 per prevalent case-year, 1998-2002, respectively)[2], plus the assumption that the value R=0.184 is under the scenario of 10% condom use with 90% protection efficacy among sexually active persons with multiple partners, the condom use rate level which is required to control HIV epidemic can be calculated as below:

Hypothetical R under 0% condom use scenario = 0.184/(1-(10%)*(90%)) = 0.202

Hypothetical R under x % condom use scenario = 0.202*(1-(x%)*(90%))

Solve the x% when R = m = 0.046; we get x% = 85.8%

In another words, the condom use rate must be increased to 86% in order to turn the tide of the sexually transmitted HIV epidemic. Sensitivity analysis to assume baseline condom use rates of 5%, 15%, and 45% only shifts the required condom use rates for HIV control to 85%, 87%, and 95%, accordingly. Thus, an equally high rate of condom use among sexually active persons with multiple partners is required to control the HIV epidemic in a low prevalence area like Taiwan in this era of highly active antiretroviral therapy.

Chi-Tai Fang,1 Yu-Yin Chang,2 Jung-Der Wang1,2

1 Department of Internal Medicine, National Taiwan University Hospital
2College of Public Health, National Taiwan University.

We declared no competing interests.

References:

1.Brown T, Peerapatanapokin W. The Asian Epidemic Model: a process model for exploring HIV policy and programme alternatives in Asia. Sex Transm Infect 2004; 80 (Suppl 1): 19-24.

2.Fang CT, Hsu SM, Twu SJ, et al. Decreased HIV transmission after a policy of providing free access to highly active antiretroviral therapy in Taiwan. J Infect Dis 2004; 190: 879-885.

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