The majority of the immunocompromised travelers in this study who received the yellow fever vaccine were HIV-positive people with CD4 count ≥ 500 cells/mm3; this practice is consistent with CDC guidelines, which consider asymptomatic HIV infection and HIV infection with CD4 count of 200–499 cells/mm3 to be a precaution but not a contraindication for yellow fever vaccine administration.9 Many immunocompromised and immunocompetent travelers in this study refused influenza vaccination. Here, CD4 is linked to influenza.