Breast cancer risk and "delayed" primary Epstein-Barr virus infection.

Parallel to its established causal association with both infectious mononucleosis (IM) and young adulthood Hodgkin disease (YAHD), we propose a hypothesis that "delayed" primary EBV infection (i.e., primary infection occurring during adolescence or adulthood) is associated with elevated breast cancer risk. We evaluated this hypothesis with two investigations, one descriptive and the other analytic. The descriptive study used international/United States cancer registry data to assess the association between incidence rates of breast cancer and those of YAHD. The incidence rates of the seemingly unrelated neoplasms were strongly correlated (correlation coefficients of 0.74 and 0.88 for international and United States data, respectively; these were higher than the correlation coefficients of YAHD with two other cancers that we considered). Populations with higher incidence rates corresponded to those with higher likelihood of delayed primary EBV infection. The analytical study was based on a population-based case-control study of breast cancer in middle-aged women. Age-adjusted odds ratios of breast cancer in women who reported a history of IM, relative to women who did not, increased monotonically from 0.55 [95% confidence interval (CI), 0.05-6.17] for women with 0-9 years of age at IM onset to 2.67 (CI, 1.04-6.89) for women with > or =25 years of age at IM onset (P = 0.016). An older age at tonsillectomy, another surrogate of delayed EBV exposure, was also associated with increased risk of breast cancer: odds ratios, 0.92 (CI, 0.57-1.48) and 1.76 (CI, 1.15-2.69) for women with tonsillectomy at 0-4 years of age and > or =15 years of age, respectively (P = 0.018). Adjusting for additional potential confounders did not modify the associations appreciably. The implications of the findings and a potential biological mechanism are presented.