The association of infectious mononucleosis and invasive breast cancer in The Health of Women (HOW) Study.

Background: The link between Epstein-Barr Virus (EBV) and breast cancer (BC) etiology remains unclear. We utilized the Health of Women (HOW) Study to understand the association between infectious mononucleosis (IM), a surrogate for EBV infection, and invasive BC.

Methods: The HOW Study was a web-based survey of BC risk factors with > 40, 000 participants; 183 had IM at < 10 years old, 3, 654 had IM between 10 and 22 years old, 764 had IM at > 22 years old, and 17, 026 never developed IM. Of these 21, 627 women, 2093 had Stages I-III BC and 14, 143 were cancer-free. Binary logistic regression ascertained the association between IM and invasive BC risk by controlling for confounders.

Results: A history of IM was associated with a lower likelihood of developing invasive BC compared to women who did not develop IM (adjusted OR = 0.83, 95% CI 0.72-0.94). That finding was driven by women who had IM between 10 and 22 years old (adjusted OR = 0.83, 95% CI 0.72-0.97) albeit no linear association between age at developing IM and breast cancer (p-trend > 0.05). Women who had IM between 10 and 22 years old were less likely to develop estrogen receptor positive (ER+ ; adjusted OR = 0.84, 95% CI 0.71-0.99) or hormone receptor positive (HR+ ; adjusted OR = 0.86, 95% CI 0.73-1.01) BC. There was no association between IM and ER- or HR- BC.

Conclusion: In the HOW Study, women diagnosed with IM between the ages of 10 and 22 had a lower risk of developing invasive BC compared to women who never developed IM.