- We evaluated the formation of hemagglutination-inhibition (HI) antibodies in response to vaccination of 55 allogeneic and 23 autologous hematopoietic stem cell transplantation (HSCT) recipients with 3.75 μg inactivated influenza A/California/7/2009 (H1N1)v-like virus adjuvanted with AS03, given towards the end of the 2009 influenza pandemic. The 78 HSCT recipients, aged 11–72 (median 50) years, were vaccinated 1–290 (median 27) months post-HSCT. Of the 55 allogeneic HSCT recipients, 50.9% received reduced intensity conditioning, 74.5% had a sibling donor, 67.2% had active graft-versus-host disease and 43.6% were on steroid therapy. At baseline, 14/78 (17.9%) had HI titers ≥1:40. Blood samples of 77 patients were available post-1st vaccination; of these, 34 (44.2%) patients had HI titers ≥1:40. Blood samples of 43 patients were available post-2nd vaccination; of these, 21 (48.8%) had HI titers ≥1:40. There was a significant increase in HI titers ≥1:40 from baseline to both post-1st and 2nd vaccinations (p < 0.001 each), and also from 1st to 2nd vaccination (p = 0.008). In seronegative (HI titers <1:10) patients, whose sera were available before, after one dose, and after 2 doses of vaccine, seroconversion (to ≥1:40) occurred in 4/24 (16.7%) after 1-dose and in a total of 10/24 (41.7%) after 2-dose vaccination (p = 0.031). Logistic regression analysis revealed that ≥1:40 HI titers were significantly associated with higher lymphocyte counts and higher HI baseline titers and, in allogeneic HSCT, with having a sibling donor and higher baseline titers. In conclusion, 2-dose vaccination with AS03-adjuvanted vaccine containing 3.75 μg antigen resulted in a statistically significant, yet limited, serological response. Therefore, additional precautions should be taken during influenza outbreaks.