Background: Vaccination against seasonal influenza virus is generally recommended for cancer patients undergoing chemotherapy because influenza infection often results in severe complications. However, it is unclear whether the immune response to vaccination is sufficient in cancer patients. Two-time vaccination is recommended in children. We are now conducting a prospective study of two-time vaccination in cancer patients undergoing chemotherapy. Here we report the effects of the first vaccination.Methods: We administered 2013/14 influenza vaccine with a triple-antigen (H1N1/H3N2, B) to cancer patients who were receiving chemotherapy. Blood samples were taken prior to vaccination and 4 ± 1 weeks after each vaccination to monitor the immune response using hemagglutination inhibition (HI) assays. HI titers >40 U/L are considered as protective against influenza infection.Results: From Nov. 2013 to Jan. 2014, 74 patients with various solid cancers (16 breast, 11 colorectal, 8 biliary tract, 7 pancreatic, 6 GIST, 6 unknown primary, and 20 others) were enrolled. The median age was 61 (range 21-89) years. Before vaccination, 36 (49%), 25 (34%), 10 (13%), and 0 patients had protective titers against 0, 1, 2, and 3 antigens, respectively. After the first vaccination, only 14 (19%) patients achieved protective titers against all 3 antigens while no protective titers were obtained against any of the antigens in 7 (9%) patients. Protective titers against 1 and 2 antigens were achieved in 20 (27%) and 28 (38%) patients, respectively. Protective titers against all 3 antigens were achieved in 7/36 (19%), 4/25 (16%), 3/10 (30%) patients who had been positive for titers against 0, 1, 2 antigens, respectively, before the vaccination.Conclusions: Although routine influenza vaccination is recommended for cancer patients receiving chemotherapy, a lower proportion of patients achieved protective titers than did healthy individuals. The efficacy of the second vaccination will be investigated.