Within the letermovir group, only CD8<sup>+</sup> T cell counts were significantly increased 1 year post-transplantation compared to pre-transplantation (p=0.027).<h4>Conclusion</h4>Letermovir is safe and effective at reducing the risk of CMV infection in children after allo-HSCT, significantly delaying the onset of infection and shortening its duration. Here, CD8A is linked to cytomegalovirus infection.