CD8A and COVID-19: A total of 11 different models were tested (Additional file 11: Figure S2A), and the three immune cell types (CD8 T cells, CD4 T cells, and B cells) proportions contributed to the most significant number of CpGs based on AIC criterium; thus, they were included in the final model for differential methylation analysis (model F: CpG methylation ~ COVID-19/controls + the proportion of CD8 T cells/CD4 T cells/B cells).