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

Background. The link between Epstein-Barr Virus (EBV) and breast cancer (BC) remains unclear. Infectious mononucleosis (IM) is a clinical manifestation of delayed onset of EBV infection in early adulthood. We utilized the Health of Women (HOW) Study to understand the association between IM and BC risk. Subjects and methods. The HOW Study was a web-based survey of BC risk factors with >40,000 participants who answered seven modules between 2012 and 2015; 3,654 women had IM between the ages of 10 and 22 years (16.8%) and 17,026 never developed IM (78.5%). Of these 20,680 women, 1,997 (9.7%) had Stages I-III BC and 13,515 (65.4%) were cancer-free. Multivariable binary logistic regression ascertained the association between IM and BC risk by controlling for ethnicity, family history, age at menarche, oral contraceptive use, tobacco use, birthplace, parity, age at first birth, body mass index, and breast biopsy. Secondary analyses stratified cancer cases into those who had BC at <50 or [≥]50 years old and by estrogen receptor (ER) subtype. Results. Participants were mostly white, middle-aged women born in the United States or Canada. Women who had IM were less likely to develop BC than those who did not develop IM (adjusted odds ratio (OR)=0.83, 95% confidence interval (CI) 0.71-0.96). Findings were similar when stratifying women into <50 or >=50 years old at BC diagnosis (<50 years old, adjusted OR=0.82, 95% CI 0.67-0.998; >=50 years old, adjusted OR=0.83, 95% CI 0.69-1.00). Women who had IM were less likely to develop ER positive BC (adjusted OR=0.84, 95% CI 0.71-0.997); there was no association between IM and ER negative BC (adjusted OR=0.88, 95% CI 0.65-1.16). Conclusion. In the HOW Study, women diagnosed with IM between the ages of 10 and 22 had lower breast cancer risk compared to women who never developed IM.