Kyeen Mesesan Andersson, M.D. ’07, Ph.D. ’07, has always liked math. She knew from childhood, too, that she wanted to be a scientist. At Brown University she began her undergraduate studies with physics and switched to molecular biology. Then, just before graduation, a “life-changing” class in vaccine development set her on a path that led through infectious disease research and medicine before returning her full circle to math.

First stop: two years as an emerging infectious diseases fellow with the Centers for Disease Control and Prevention, where Andersson worked as a virologist on the dengue fever vaccine development team, devising mutant clones that might weaken the virus and be incorporated into a vaccine. At a conference, she met an official in the U.S. Army Medical Research Institute of Infectious Diseases who offered to send her to Peru so she could gain clinical experience with the disease.

She spent a month in the Amazon port of Iquitos, where she saw many people infected by the virus she knew so well from the lab. Seeing the clinical picture, she became fascinated by the possibility of bridging the gap between science and medicine. That clinched her decision to apply to M.D./Ph.D. programs.

Andersson entered the School of Medicine in 1999. At an AIDS conference during medical school she discovered a third approach to infectious disease: one of the speakers was Sir Roy Anderson, Ph.D., a British epidemiologist known for his predictive mathematical models of Creutzfeldt-Jakob disease and AIDS. “I remember being absolutely amazed that you could use math to forecast disease growth and disease processes,” Andersson said. She was hooked.

Under faculty advisors David A. Paltiel, Ph.D., professor of public health, and Linda M. Niccolai, Ph.D., associate professor of epidemiology, Andersson spent three years in South Africa creating a mathematical model of the HIV epidemic in Soweto as well as surveying adults on their sexual risk-taking behaviors. Her aim was to predict whether changes in behaviors that resulted from taking an HIV vaccine would have an effect on the HIV epidemic.

Policymakers deciding on new disease interventions can’t wait the years that it takes to collect and analyze data, so they may rely instead on mathematical simulations, Andersson explained. No one knows, for instance, whether people who have received a partially effective HIV vaccine would take more or fewer precautions, nor the effects their post-vaccination behaviors might have on the virus’ spread. Andersson’s Ph.D. thesis created a model to answer just such questions. A vaccine that’s merely 30 percent effective, she found, could still save lives. Three large clinical trials in Africa had shown that male circumcision could reduce transmission of HIV from women to men. Intrigued by those findings, she also modeled male circumcision and HIV transmission in Soweto, and found that an adult male circumcision program could greatly reduce the expected number of new infections. Those results made world headlines after she presented them at the 16th International AIDS Conference in Toronto in 2006.

After graduating from Yale she signed on as a postdoctoral fellow at the School of Public Health and explored the gender dynamics of HIV-preventive circumcision programs. Her new model showed that while widespread male circumcision could reduce new infections, it could also increase new infections among Soweto women. The reason? Circumcised men may skip condoms—and women there may lack the power to demand their use.

Since 2010, Andersson has continued developing mathematical models at the Futures Institute in Glastonbury, Conn., for clients like the Gates Foundation, UNAIDS, and health ministries overseas. She and her husband Richard, a South African engineer who works at Yale’s Office of Cooperative Research, are the parents of 2-year-old Amelia and 1-year-old Alexis.

Partly because of her four-year delay between college and medical school, Andersson, 39, decided to forgo residency training. “In an ideal world, I would have done both clinical medicine and research,” she said. “I chose to do what was closest to my heart.” She had once asked her mentor at Brown, Anne De Groot, M.D., how she juggled research, family, and a medical practice.

“That’s when I discovered that she had an ex-pro-hockey-player husband who was raising both kids and keeping house,” Andersson recalled, laughing.

Though she misses clinical care, Andersson loves the power of mathematical modeling. A recent question from one of the institute’s clients underscored that for her.

“When Bill Gates comes and asks you, ‘How much is it going to cost to treat everyone in the world [who has HIV]?’ ” she noted, “it’s probably for a good reason.”