Journal of Vaccine Research 2024, Vol.14, No.3, 107-119 http://medscipublisher.com/index.php/jvr 112 strategies to combat hepatitis B. This includes ensuring access to vaccines, improving vaccination coverage, and monitoring the impact of immunization programs (Locarnini et al., 2015; Hutin et al., 2018). While significant progress has been made in the development and implementation of hepatitis B vaccines, continued efforts are needed to address the remaining challenges. Sustainable vaccination programs, improved coverage, and targeted vaccination efforts for high-risk communities are critical to achieving the global elimination of HBV transmission (Shepard et al., 2006; Zanetti et al., 2008; Locarnini et al., 2015; Hutin et al., 2018; Zhao et al., 2020; Pattyn et al., 2021; Tall et al., 2021). 5 Vaccine-Induced Immunity and Challenges 5.1 Immunological response to HBV vaccines The immunological response to hepatitis B virus (HBV) vaccines is primarily measured by the production of antibodies against the hepatitis B surface antigen (anti-HBs). Effective vaccination is defined by the induction of protective anti-HBs titers, which are indicative of immunity. Studies have shown that the magnitude of the immune response can be influenced by the type of vaccine used. For instance, a tri-antigenic HBV vaccine (TAV) has been demonstrated to elicit significantly higher anti-HBs titers compared to a mono-antigenic HBV vaccine (MAV) (Langley et al., 2020). This higher antibody response is crucial as it is believed to predict the persistence and durability of seroprotection. In populations such as healthcare students, who are at high risk of exposure, the persistence of humoral immune protection induced by the primary cycle of hepatitis B vaccination has been shown to be high, with a seroprotection prevalence of 73.8% and an anamnestic response rate of 90.9% following a challenge dose (Rahmani et al., 2022). This indicates that the majority of individuals maintain effective immunity for over two decades post-vaccination. 5.2 Factors influencing vaccine efficacy Several factors can influence the efficacy of HBV vaccines. Age, gender, body mass index (BMI), smoking status, and the presence of concomitant diseases have all been identified as significant determinants of vaccine response. For example, adults aged 40 years and older, males, individuals with a BMI of 25 or higher, smokers, and those with concomitant diseases exhibit significantly decreased responses to the hepatitis B vaccine (Yang et al., 2016). Additionally, the timing of vaccination in relation to disease status is critical. In patients with inflammatory bowel disease (IBD), vaccination during disease remission and before the initiation of immunosuppressive therapy is associated with better immune responses (Jiang et al., 2017). Host genetic factors also play a role in vaccine-induced immunity. Variations in genes such as IFNG, MAPK8, and IL10RA have been associated with differences in peak anti-HBs levels, which are directly related to the durability of the antibody response and long-term vaccine efficacy (Hennig et al., 2008). 5.3 Overcoming immunological challenges To overcome the immunological challenges associated with HBV vaccination, several strategies have been proposed. One approach is the use of higher vaccine doses or additional booster doses. For instance, a double dose of the HBV vaccine and a four-dose schedule have been shown to result in better immune responses compared to the standard three-dose regimen, particularly in people living with HIV (PLWH) (Figure 2) (Tian et al., 2021). Similarly, high-dose vaccines have been found to provide more satisfactory long-term immunogenicity in hemodialysis patients (Yao et al., 2021). Another strategy involves the use of adjuvants to enhance the immune response. The use of Fms-like tyrosine kinase 3 ligand (Flt3L) as an adjuvant in DNA vaccines has been shown to significantly enhance both humoral and cellular immune responses, suggesting potential improvements in vaccine efficacy (Zhou et al., 2010). Therapeutic vaccines are also being developed to induce robust T-cell responses in chronic HBV patients. A novel vaccine formulation comprising particulate hepatitis B surface and core antigens, combined with the ISCOMATRIX™ adjuvant, has demonstrated the ability to break immune tolerance and induce potent T-cell
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