We thank S. Watson for scientific editing. We also thank N. Midorikawa for technical assistance. In addition, we thank E. Adachi, T. Koibuchi, T. Kikuchi, M. Koga, H. Yotsuyanagi, A. Tokita and N. Wada for providing us with the clinical specimens from patients with influenza-like symptoms. Flow cytometry was performed in the IMSUT FACS Core laboratory; we acknowledge the IMSUT FACS Core laboratory for assistance with the flow cytometric analysis. This research was supported by Leading Advanced Projects for medical innovation (LEAP) from the Japan Agency for Medical Research and Development (AMED) (JP18am001007), by Grants-in-Aid for Scientific Research on Innovative Areas from the Ministry of Education, Culture, Science, Sports, and Technology (MEXT) of Japan (nos. 16H06429, 16K21723 and 16H06434), by the Japan Initiative for Global Research Network on Infectious Diseases (J-GRID) from AMED (JP18fm0108006), by an e-ASIA Joint Research Program from AMED (JP17jm0210042), by a Research Program on Emerging and Re-emerging Infectious Diseases from AMED (JP18fk0108104), by a Grant-in-Aid for JSPS Research Fellows (17J04123) and by the NIAID-funded Center for Research on Influenza Pathogenesis (CRIP; HHSN272201400008C).