Our study identified 2 immune cell subsets, specifically IgD- CD27- AC and CD27 on IgD + CD38- unsw mem cells, as mediators linking critically ill COVID-19 to HER2-positive breast cancer, indicating that dysregulation of these immune cell subsets may be a possible mechanism through which critically ill COVID-19 influences HER2-positive breast cancer. This evidence concerns the gene CD38 and breast cancer.