In A(H1N1)pdm09 infected patients, PD-L1 expression on CD8+ T cells is inversely correlated with CD4+ and CD8+ T cell proportions, but this correlation was only observed in pH1N1+ patients; this finding could be explained by the fact that the lymphopenia induced by A(H1N1)pdm09 has been reported to be more severe and refractory than that associated with seasonal infection, which is modest during the first days and resolves earlier [31]. Here, CD274 is linked to lymphopenia.