THE PROBLEM
HIV is an epidemic with varying methods of transmission accounting for its spread worldwide. Heterosexual contact is the primary source of HIV transmission in Africa, and the major HIV transmission mode in Asia, which accounts for 60% of the world's population.

"Homosexual transmission and shared needles among injecting drug users are still the primary sources of transmission in North America, but the proportion of new AIDS cases accounted for by heterosexual transmission is increasing steadily," Satten said. A unique study group of male military inductees in Thailand provided an opportunity for Dr. Satten and his colleagues to develop models of the probability of female-to-male HIV transmission per sexual contact.

The analysis was possible because the situation in Thailand allowed for the assumption that HIV-1 men in the study had been infected through sex with female prostitutes since 1988. "The Thai study group is unique because we can determine how and when the HIV was most likely transmitted, then chart the infection from its onset and continue as it progresses through stages," said Satten. "Identifying the stages of HIV development will assist in treatment of the infection."

While prostitution in Thailand is not legal, it is socially accepted. The military inductees provided information on their frequency of contacts with prostitutes, which would be difficult to obtain in other settings. In addition, the Thai Ministry of Public Health has been monitoring the increase of HIV among female prostitutes since mid-1989, when the prevalence was 3.2%. By mid-1992, the prevalence had increased to 23%.

THE SOLUTION
"The Thai study is unique because we know the mode of transmission can determine the number of contacts of which transmission could have occurred, but most important because these contacts occurred with people who were likely to be recently infected, since widespread transmission of HIV in Thailand was a fairly recent phenomenon at the time the study was conducted. Our study supports the hypothesis that individuals who are recently infected are themselves more infectious," said Satten.

A Fortran programmer, Dr. Satten first used the IMSL Libraries during postdoctoral work in biostatistics at the University of North Carolina at Chapel Hill in 1988. Models he develops at the CDC rely primarily on the optimization routines in the IMSL Libraries to estimate the values of parameters that best describe the observed data, which are called maximum likelihood estimates. The CDC relies primarily on an IBM mainframe and PCs, although Dr. Satten has recently received authorization to obtain a DEC® Alpha 3000-600 workstation to run both the IMSL Fortran Numerical Libraries and IMSL Exponent Graphics software. Increased speed is the primary reason for his move to a UNIX®-based system.

RETURN ON INVESTMENT
"I use the IMSL Libraries to reduce my program development time so that I can concentrate on those aspects of a problem that are novel and require statistical or AIDS-related expertise," says Satten. The IMSL Libraries were used to model the data, and the findings were presented to researchers at the IX International Conference on AIDS in June 1993 in Berlin. A mathematical model was developed to estimate the probability of HIV-1 transmission per sexual contact. Data included the inductee's age at first sexual contact, frequency of sex with female prostitutes, province of origin and province-specific HIV-1 seroprevalence among prostitutes.