T follicular helper (TFH) cells, a critical subset of CD4+ T cells, provide help to B cells during the procession of the humoral immune response in the germinal center (GC) and extrafollicular sites. CXCR5+ CD4+ T cells in human circulating blood, referred to herein as peripheral TFH (pTFH) cells, share phenotypes and functional properties with TFH cells in GC. Hepatitis B vaccine protects about 60% of the chronic hepatitis C patients from hepatitis B. The immunological bases that lead to the induction of protective antibody response is not well understood. In the present study, the pTFH cells subsets were determined in 18 healthy controls (anti-HBs ≥ 100 mIU/mL; HC), 21 nonresponders (anti-HBs < 10 mIU/mL; NR), and 23 weak responders (10 mIU/mL ≤ anti-HBs < 100 mIU/mL; WR) of chronic hepatitis patients upon routine hepatitis B vaccination. Though the frequency of the pTFH cell was equivalent in HC, WR, and NR, ICOS+ pTFH cells in HC underwent expansion with increased IL-21 secretion and production of serum anti-HBs response at 4 weeks after a full course of hepatitis B vaccination. These changes were not shown in both NR and WR. Analysis of ICOS+ pTFH cells represents a novel cellular determinant of the hepatitis B vaccine-induced humoral immune response, which may have relevance for design of hepatitis B vaccine. © 2019 Wiley Periodicals, Inc.