Our findings demonstrated that at 5 days post-infection (dpi), the counts of B cells, CD4+ T cells, and CD8+ T cells were significantly higher in mice intravenously administered with inactivated E. faecium combined with inactivated H1N1 virus compared to those intramuscularly administered with the same formulation, indicating that intravenous administration enhances adaptive immunity. This evidence concerns the gene CD8A and infection.