Background: Breast cancer (BC) mortality in South Africa (SA) is double that of the United States. The aggressive triple-negative breast cancer (TNBC) subtype comprises 20.9% of BC cases among Black women in Southern Africa. SA has the second highest HIV prevalence worldwide, but if and how HIV affects the risk of developing TNBC is not well understood. Methods: Using the South African Breast Cancer and HIV Outcomes (SABCHO) prospective cohort of newly diagnosed BC patients from six public hospitals in SA, we evaluated the association of HIV with risk of TNBC relative to other BC subtypes. We developed multivariable logistic regression models to test for this association while adjusting for key demographic and reproductive risk factors. Results: Of 3,883 women with BC, 16.4% had TNBC and 23.0% were HIV-positive. The overall median age of BC diagnosis was 55 years. Among HIV positive patients, the median age of BC diagnosis was 45 years, without a difference between TNBC and non-TNBC cases (p = 0.244). Only 7.2% of women were nulliparous (4.6% in TNBC and 7.6% in non-TNBC subgroups). Of women who had at least 1 full-term pregnancy, 11.2% did not breastfeed, without a difference between TNBC and non-TNBC cases (p = 0.292). In our final multivariable logistic regression model, HIV-positive status was associated with increased risk of TNBC relative to non-TNBC (OR: 1.40; 95% CI: 1.12-1.74, compared to HIV-negative with BC). Parity without breastfeeding was also associated with increased risk of TNBC relative to non-TNBC (OR: 1.72; 95% CI: 1.07-2.84, compared to nulliparous with BC). HIV status and the combined parity/breastfeeding variable had no statistically significant interaction. In exploratory analyses, measures of HIV control at time of BC diagnosis were not associated with BC subtype. Conclusions: In this large prospective case-only study conducted in SA, we found that women living with HIV were more likely than HIV-negative women to have TNBC (relative to other BC subtypes), regardless of the extent of HIV control. Whether this relationship is mediated by immune mechanisms or other factors should be the focus of future research. Our findings highlight the importance of rigorous BC screening and early detection among HIV-positive women given their increased relative risk of developing TNBC.

*Bolded odds ratio indicates p-value < 0.05; ^¡Model adjusted for age.