The Sustainable Goal of Eliminating Hepatitis B, Functional Cure, and the Prospect of NASVAC: A Review

Abstract

Hepatitis B is a global pandemic, being one of the rare infectious causes of the rising number of deaths globally, whereas these numbers are on the decline with all other major infections like malaria, tuberculosis, and HIV/AIDS, to name a few.

We have useful antivirals against the hepatitis B virus (HBV) that can contain the disease, but we are still far away from the Holy Grail that will cure the disease once and forever. Having said so, this also has to be kept in mind that both the World Health Organization and the Sustainable Development Goal call for the effective elimination of this menace by 2030, which is a few years from now.

The race is therefore on for developing the right cure for hepatitis B, and a therapeutic vaccine is one such candidate in the growing list of potential future cures for the virus.

Here we discuss NASVAC, an immune-therapy for chronic hepatitis B, which is established as safe and efficacious against the virus and may well be the option that we are so eagerly looking forward to for achieving functional cure of hepatitis B to benefit the missing millions.

How to cite this article

Mahtab MA, Rubido JCA, Guillen GE, et al. The Sustainable Goal of Eliminating Hepatitis B, Functional Cure, and the Prospect of NASVAC: A Review. Euroasian J Hepato-Gastroenterol 2025;15(2):178–184.

Functional Cure for Hepatitis B Virus (HBV)

The definition of functional cure for HBV infection varies. The more or less accepted definition states that to label someone as a functional cure from chronic hepatitis B (CHB), one has to have sero-clearance of HBsAg in addition to sustained HBV DNA suppression and persistent HBeAg seroconversion following a finite duration of therapy.^1–3 While the Asian Pacific Association for the Study of the Liver (APASL) is currently working on a consensus document to define functional cure of HBV, the Europeans and Americans have already agreed upon their definition. European Association for the Study of the Liver (EASL) and American Association for the Study of Liver Diseases (ALSDB) define HBV functional cure as HBsAg negative, serum HBsAg below 0.05 IU/mL, with or without HBsAg seroconversion, i.e., anti-HBs positive, HBeAg negative or anti HBe positive, serum HBV DNA <10 IU/mL, normal serum ALT, with or without cccDNA presence, integrated DNA presence, improved clinical outcome, very low residual risk of HCC, durability over time, absence of stigma for HBV and no or low infectivity at 24 weeks off treatment.⁴

Importance of Functional Cure

In 2015, the World Health Organization (WHO) announced the target of “elimination of hepatitis (B and C) by 2030.” The target is to reduce new HBV infections by vaccination, harm reduction, and public health measures.⁵ However, the challenge in achieving this WHO as well as sustainable development goal (SDG), lies elsewhere. Of the 296 million chronic HBV-infected global population, approximately 12–25%, i.e., 36–60 million, are in need of immediate treatment to prevent progression to liver cirrhosis (LC) and hepatocellular carcinoma (HCC).⁶ At this moment, only <1% or 2–3 million CHB patients are under treatment, and that too in the developed countries.⁶ On the contrary, the bulk of the CHB patients in the underdeveloped and developing countries are even unaware that they are harboring the disease. On one hand, the WHO goal can never be achieved unless we achieve a functional cure for HBV, while on the other hand, we don't yet have the right option to attempt for functional cure of HBV in our hands. Thus, therefore, a new and innovative therapy for HBV is time demanding.⁷

Achieving Functional Cure for HBV: Where do we Stand?

Achieving functional cure in HBV is extremely crucial as it has been shown that HBsAg loss leads to regression of hepatic fibrosis, lower risk of LC, decompensation, and liver transplantation.^8–11 In fact, HBsAg loss leads to a reduction in both liver-related and all-cause mortalities.^9–11 Although the importance of functional cure of HBV is well recognized, the means to achieve this ambitious goal remains beyond our reach. Currently available drugs for treating HBV include neucleos(t)ide analogues (NA), which inhibit HBV polymerase and block replication of HBV DNA activity, and pegylated interferon (PEG-IFN)-α2a and -α2b, through antiviral and immunomodulatory effects.^12,13 However, both NAs and PEG-IFNs have limitations. Long-term treatment with NAs suppresses HBV DNA well and also improves survival and lowers the risk of HCC, they are inefficient in reducing HBsAg and cccDNA.^12,13 They need to be used for a long time, at times for life, and can result in viral resistance and adverse events like reduction of bone mineral density (BMD) and renal impairment.^14,15 Compared to NAs, PEG-IFNs are associated with higher HBsAg loss and HBeAg seroconversion.¹⁶ However, PEG-IFN appears to benefit a small segment of HBV-infected population, namely, those with low quantitative HBsAg, children, adolescents, and HBV genotype A, which is uncommon in the Asia Pacific, home to the bulk of the world's HBV-infected population.^17,18 Besides, PEG-IFN is associated with a number of adverse events, namely, fatigue, headache, myalgia, flu-like syndrome, neutropenia, thrombocytopenia, and abnormal ALT levels.¹⁹ PEG-IFN is contraindicated in pregnancy, in patients with pregnant partners, depression, schizophrenia, epilepsy, autoimmune diseases, decompensated liver disease, retinal disease, severe infection, heart failure, chronic obstructive pulmonary disease (COPD), and relatively contraindicated in thyroid disease, uncontrolled hypertension, uncontrolled diabetes mellitus, and other heart diseases.¹⁹

Therefore, the quest for the right agent to ensure a functional cure for HBV is ongoing. Several approaches are being explored. The ones that appear to be promising include anti-sense oligonucleotide (ASO), small-interfering RNA (siRNA), capsid assembly modulators (CAMs), HBsAg inhibitors, checkpoint inhibitors, entry inhibitors, toll-like receptor agonists, and therapeutic vaccines.^20–29

In this review, we will discuss a potential therapeutic vaccine that shows promise in inducing a functional cure for HBV. It is well recognized that compared to those with acute hepatitis B in whom the infection resolves spontaneously, in CHB, HBV-specific host immune response is weak.^30,31 This understanding paved the way for the exploration of immunomodulators for treating HBV. Attempts with polyclonal immunomodulators like cytokines, growth factors, and non-antigen-specific immune activators, including PEG-IFN, have been unsuccessful either due to significant adverse events or poor therapeutic efficacy.³² Subsequently, HBsAg-based vaccines, a combination of HBsAg-based vaccines and antiviral drugs, and HBsAg-pulsed antigen-presenting dendritic cells (DC) were evaluated.^33–38 Although HBsAg-based vaccines are safe, their efficacy and the sustainability of the antiviral effect remain questionable. An immunotherapeutic approach with multiple HBV antigens administered through a modified route may prove to be game-changer.²⁶

However, developing an immune-therapy for HBV has remained challenging for a number of reasons. HBV is a non-cytopathic virus, and aberrant host immunity is held responsible for the HBV-related liver damage, persistence of HBV replication, and development of HBV-related complications.^39,40 Several additional factors are also responsible for the reason why an effective immune-therapy for CHB is yet to be in our hands. For example, in most cases, pre-clinical animal studies have failed to generate sufficient evidence to translate immunotherapy from bench to bedside.⁴¹ Besides, in most instances, animal studies were designed as pilot or observational ones and not for providing long-term follow-up data to assess their therapeutic implications. Also, no or very little information is available in the literature regarding their mechanism of action.⁴¹

The first evidence of the beneficial role of two HBV antigens, i.e. HBsAg and HBcAg, specific immune response was evident when it was seen that CHB patients cleared HBV and developed ant-HBs when they received bone marrow cells from naturally protected donors expressing both HBsAg- and HBcAg-specific immunocytes.⁴² It has also been demonstrated that increased intrahepatic HBcAg-specific cytotoxic T cells (CTL) contain HBV replication and liver damage in CHB patients.⁴³ Besides, in HBV-transgenic mice (HBV TM), a model of chronic HBV carrier state, it has been observed that HBcAg has an adjuvant-like effect and induces sustained HBV-specific safe antiviral activity.⁴⁴ All these observations justify the evaluation of a therapeutic vaccine containing both HBsAg and HBcAg in CHB patients. NASVAC is a therapeutic vaccine containing both these antigens. The safety and efficacy of NASVAC were assessed in both HBV-transgenic mice and healthy human volunteers.^45–47

NASVAC is designed, produced, and developed by the Centre for Genetic Engineering and Biotechnology (CIGB), Havana, Cuba. It contains 100 µgm Pichia pastoris-derived recombinant HBsAg subtype adw2 and 100µgmEscherichia coli-expressed recombinant full-length HBcAg, GenBank accession number X02763.²⁷ NASVAC is administered intra-nasally, as nasopharynx-associated lymphoid tissues (NALT) in the nose elicit both local mucosal as well as strong systemic immunity.^48,49 A multi-dose nasal sprayer (VP7D, Valois, France) calibrated to dispense 125 µL with every push on the plunger is used for intra-nasal delivery of NASVAC.²⁶ Mechanism of action of NASVAC is summarized in Table 1.^26,50–53

Table 1.

Summary of key mechanisms of action of NASVAC

Effect on	Mechanisms of action	Pharmacological outcome
Dendritic cell activation	NASVAC activates dendritic cells (DCs), which are crucial for initiating immune responses.	High levels of anti-HBs, HBsAg-specific, and HBcAg-specific T-cells were detected in the spleen and liver of HBV-transgenic mice immunized with NASVAC.48
B- and T-cell activation	NASVAC stimulates both B- and T-cells, enhancing adaptive immunity.	B-cell activation markers increased in CHB patients after in vitro stimulation with NASVAC, and CD4+ and CD8+ T-cells showed increased expression of activation markers.49
Cytokine secretion	NASVAC induces the secretion of pro-inflammatory cytokines, which are essential for antiviral defense.	PBMC from vaccinated CHB patients produced significantly higher levels of cytokines (IL-1β, IL-6, IL-8, IL-12, and TNF-α) compared to unvaccinated.50
Nasal route advantage	The intranasal route of administration enhances immune responses, particularly in the liver.	The intranasal route was the most effective at inducing CD4+ T-cell responses in the liver, with high frequencies of multifunctional CD4+ T-cells secreting IFN-γ, IL-2, and TNF-α.51
Multi-TLR agonist effect	HBcAg acts as a multi-toll-like receptor (TLR) agonist, stimulating innate immunity.	HBcAg activates TLR2, TLR3, TLR7, TLR8, and TLR9, leading to increased expression of HLA class I/II and co-stimulatory molecules, as well as type I interferons and cytokines.52

A phase I/II clinical trial of NASVAC was conducted in 18 treatment-naïve CHB patients in Bangladesh by our group.²⁶ Therapeutic vaccination with NASVAC was conducted in 2 cycles. In the first cycle, the vaccine was administered intranasally on 5 occasions at 2-weekly intervals. In the second cycle, the same formulation of NASVAC was administered both intranasally and subcutaneously using the nasal spray following the same schedule. There was an interval of 4 weeks between the 2 cycles.²⁶ Results showed that although ALT was above the upper normal level (UNL) in all patients, 16 and 18 patients had normal ALT at the end of treatment (EOT) and at 48 weeks off treatment follow up, respectively. Three out of 7 HBeAg-positive CHB patients became HBeAg negative at EOT, and the response was sustained at 48-week follow-up. Seven out of 11 HBeAg-negative and 2 out of 7 HBeAg-positive patients had undetectable HBV DNA in sera at 48 weeks. A further 4 HBeAg-positive patients were partial responders, as their HBV DNA load declined by 2–4 log copies at 48 weeks. IL-1β, IL-6, IL-12, IL-8, and TNF-α levels were significantly higher in supernatants from NASVAC-stimulated PBMC compared to those stimulated with PDC or no antigen. NASVAC-pulsed DCs produced significantly higher levels of IL-1β, IL-12, and TNF-α than PDC-pulsed DCs from NASVAC-vaccinated CHB patients and NASVAC-pulsed DCs from control CHB patients. There was no significant adverse event (AE).²⁶

We subsequently conducted a phase III clinical trial, where 160 treatment-naïve CHB patients were included in this study after screening 360 patients. The patients were randomized into two groups, each containing 80 patients. One group was treated with NASVAC, following the same schedule in the phase I/II clinical trial and the same formulation.²⁶ Patients in the other group were treated with a total of 48 weekly subcutaneous doses of PEG-IFN. At EOT, HBV DNA reduction was similar in both groups, NASVAC vs PEG-IFN being 59.0% vs 62.5%, p > 0.05. Also, the number of patients with HBV DNA <1000 copies/mL was similar, i.e., NASVAC vs PEG-IFN 69.2% vs 65%, p >0.05. However, at 24-week follow-up, a significantly higher percentage of patients (57.7%) had undetectable HBV DNA (<250 copies/mL) compared to the PEG-IFN-treated group (35%) (p < 0.05).

The post-treatment viral rebound was more profound in the PEG-IFN group. Transient and homogeneous rise in ALT was noted in 85% NASVAC-treated patients, compared to 30% in the PEG-IFN-treated group. In NASVAC-and PEG-IFN-treated groups, HBeAg seroconversion was seen in 35.7% and 18.7% patients, respectively, which in both cases was associated with a marked reduction of HBV DNA. On 24-week follow-up, 7 patients in the PEG-IFN-treated group developed LC, i.e. liver stiffness >18.3 kPa in fibroscan, which was not seen in any patient in the NASVAC-treated group. Two patients in the PEG-IFN-treated group and none in the NASVAC-treated group developed hepatic decompensation. NASVAC appeared to be much safer than PEG-IFN in all major variables related to AE.²⁷

The ALT elevation experienced in the majority of NASVAC-treated patients after five doses of nasal administration was not seen in healthy volunteers.⁴⁷ This ALT elevation possibly indicates restoration of host immunity, which may be the key to the therapeutic effect of NASVAC in CHB. In fact, in most NASVAC-treated patients, a reduction in HBV DNA followed the ALT elevation.²⁷

Patients included in the NASVAC phase III clinical trial were followed up 2, 3, and 5 years after treatment. At 2 years, 66 out of 78 patients of the phase III trial could be followed up. The rest 12 patients, although they missed the follow-up, were contacted over the telephone and were found to be healthy and had no liver disease-related symptoms. HBV DNA was under sustained control in all patients, being undetectable (<250 copies/mL) in 33 out of 66, and in another 30 patients, HBV DNA level was below the baseline level. Serum ALT was within normal limits in 47 out of 66 patients, and none had ALT, 2 times above UNL. Among these 66 patients, 12 were HBeAg positive, but at 2 years, 8 became HBeAg negative, and of these 8, 5 also developed anti-HBe in sera. Most importantly, none of these 66 patients developed LC, which was confirmed by fibroscan and ultrasonography of the hepato-biliary system.⁷

The patients were followed up again after another year. This time, 59 patients turned up for follow-up. Safety issues were evaluated in the remaining 19 patients over the telephone. Of these 59 patients, 20 patients still had undetectable HBB DNA after 3 years, and HBV DNA was below the baseline level in 36 more patients. Serum ALT was within normal limits in 42 patients and below twice the UNL in another 15. There was no evidence of progression of liver disease in any patient.⁵⁴

The patients were brought back again after another 2 years for follow-up. This time, 60 patients turned up. Of them, 45 still had undetectable HBV DNA (<250 copies/mL), while the level was reduced in 10 more, establishing the outstanding long-term antiviral potential of NASVAC. At 5 years, 40 patients had normal serum ALT levels, and 8 out of 12 HBeAg-positive patients remained HBeAg negative. However, most importantly, once again, no patient had liver disease progression.²⁸

None of the above studies of NASVAC, however, provided any data on the effect of this immunotherapy on HBsAg. This issue, which is crucial for functional cure of HBV, has been addressed in a more recent phase IIa clinical trial of NASVAC in Japan. Here, a modified version of NASVAC was used. About 800 µL aliquot of NASVAC was emulsified in 200 µL CVP (Toko Yakuhin Kogyo Co. Ltd., Osaka, Japan) to potentiate the immunogenicity of NASVAC.⁵⁵ A new nasal device (Toko Yakuhin Kogyo Co. Ltd., Osaka, Japan) was used for nasal administration of the drug. The dose of NASVAC was, however, similar to the phase I/II and III clinical trials in Bangladesh. NASVAC was applied following the previous schedule to 27 CHB patients on NA treatment and another 36 patients who were HBV carriers not on any treatment. The patients were followed up for 18 months. Twenty out of 27 and 27 out of 36 patients in the two groups, respectively, showed HBsAg reduction, while 6 patients in the CHB on NA group and 4 more in the HBV-carrier group lost HBsAg as revealed by ARCHITECT HBsAg Assay (Abbott Laboratories, Chicago, IL, USA). Eleven out of 27 patients in the CHB on NA group and another 21 out of 36 in the HBV-carrier group, respectively, showed a mean rise in anti-HBs titer by 39.9+81.1 mIU/mL and 70.4 + 145.0 mIU/mL. Preliminary results of a phase IV clinical trial have yielded even more promising results, where HBsAg loss has been found in 16% patients.⁵⁶

Functional cure was achieved in 6 out of 71 (9.5%) patients included in this clinical trial.²⁹ Annual HBsAg loss in CHB with and without NA has been shown to vary between 0.26 and 0.33% and 1.13% respectively.^57–59 The result obtained by the Japanese study is surely far more impressive than this. The HBsAg reduction rate (–0.2677 log₁₀IU/mL) in this study is also higher than that reported in the previous literature.^60–62

Among the CHB in the NA group, 2 patients who were HBV DNA positive became negative at EOT, while 19 out of 36 patients among the HBV-carriers had a reduction of HBV DNA from baseline. HBV DNA loss negativity was sustained in all 6 patients who lost HBsAg and developed anti-HBs. Anti-HBe became positive in 4 patients with CHB on NA, and anti-HBe positivity increased from 12 out of 27 to 16 out of 27 patients. HBeAg titer reduction was seen in 8 patients, while significant HBcrAg reduction was seen in all CHB patients on NA, but not in HBV-carriers. No significant AE was noted in any patient.²⁹

In vitro HBV neutralizing assay showed that plasma samples obtained from NASVAC-treated patients of this clinical trial containing high titers of anti-HBs displayed strong neutralizing capacity, while plasma from NASVAC-treated patients of the clinical trial with undetectable anti-HBs had no neutralizing capability. This indicated HBV was effectively neutralized by NASVAC-introduced anti-HBs.²⁹

HBcrAg, which is a combination of HBcAg, HBeAg, and 22-kDa pre-core proteins, represents transcriptional activity of cccDNA.⁶³ HBeAg, on the other hand, contains HB core domain peptides and is a marker of replication and viral activity of HBV.⁶⁴ Therefore, elimination of HBcrAg and HBeAg is crucial for the induction of a functional cure of HBV. Both HBcrAg and HBeAg titers were reduced following administration of NASVAC in this study.²⁹

An ongoing phase-IV clinical trial of NASVAC has yielded even promising results. At five years following treatment with NASVAC, 16.88% CHB patients lost HBsAg, while 30% patients recorded 1-log HBsAg reduction. Overall, HBsAg levels were reduced in 90% patients. The study included all 160 CHB patients.⁶⁶ A comparison between NASVAC and commercially available antivirals, i.e., NAs and PEG-IFN, is summarized in Table 2.

Table 2.

Comparison between NASVAC and antivirals for management of CHB

Aspect	NUCs	PegIFN	NASVAC
Mechanism of action	Suppress HBV replication by inhibiting viral polymerase.	Stimulates innate immunity but with significant AE.	Stimulates both innate and adaptive immunity, targeting multiple HBV antigens.
Treatment duration	Lifelong or long-term treatment required.	Finite (48 weeks) but with significant AE.	Finite (10 doses over 20 weeks), reducing long-term healthcare costs.
Safety and tolerability	Long-term complications, nephrotoxicity, bone density loss, resistance.	Significant AE: Flu-like symptoms, depression, hematologic issues.	Favorable safety profile with mild AE.
Antiviral efficacy	Strong HBV DNA suppression but limited HBsAg loss (2.5% after 10 years).	Moderate HBV DNA and HBsAg reduction (3–12% HBsAg loss).	Sustained HBV DNA suppression and significant HBsAg reduction (16% loss).55
Liver protection	Reduces inflammation, but residual HCC risk remains high.	Reduces HCC risk but is limited by significant AE.	Prevents LC and HCC, with long-term follow-up showing no progression.
Cost-effectiveness	High cost due to lifelong treatment.	High cost and significant AE limit the use in low-income settings.	Suitable for large-scale use in resource-limited settings.

NASVAC is already registered and commercially available in Cuba (HeberNasvac, Heber Biotech S.A., Havana, Cuba) and a few other countries (Fig. 1).⁶⁵ In 2024, the Hepatitis B Foundation, USA, identified NASVAC as one of the top therapeutic vaccines effective in CHB (Table 3). NASVAC now deserves much more attention from the global community in order to benefit the missing millions.

Fig. 1.

HeberNasvac (with permission obtained from Center for Genetic Engineering and Biotechnology (CIGB), Habana, Cuba⁶⁵

Table 3.

Potential therapeutic vaccines against hepatitis B

Immunologicals: Targets the human immune system to attack the HBV virus
Therapeutic Vaccine Technology used to stimulate the immune system as a treatment
HeberNasvac	Therapeutic vaccine	CIGB, Cuba	Studies in Cuba	Phase IV
VBI-2601 (BRII-179)	Therapeutic vaccine	VBI Vaccines with Brii Biosciences, USA	vbivaccines.com and briibio.com	Phase II
VVX001	Therapeutic vaccine	Viravaxx, Austria	viravaxx.com	Phase II
GSK 3528869A	Therapeutic vaccine	GSK, USA	gsk.com	Phase II
VTP-300	Therapeutic vaccine	Barinthus Biotherapeutics, UK (formerly Vaccitech)	www.barinthusbio.com	Phase II
CVI-HBV-002	Therapeutic vaccine	Cha Vaccine Institute, S. Korea	en.chavaccine.com	Phase I/II

Conclusion

NASVAC is the only immune-therapy for which follow-up data of 5 years have been published. This is a landmark development in the development of new, novel, evolving therapies. There remained concern about NASVAC-induced nasal mucosal inflammation, which proved to be incorrect. Similarly, HBcAg-induced hepatitis flare was also proven not to be an issue. NASVAC is a new, novel immune-therapy for CHB of finite duration of administration, which has been shown to induce HBsAg and HBeAg-seroconversion, sustainable control of HBV DNA and ALT, and most importantly, prevent the liver disease progression. The safety of the good manufacturing practice (GMP) level of NASVAC has also been established in HBV TM and in healthy human volunteers. NASVAC and immune-therapy offer new hope for finally achieving a functional cure for HBV and bring halt to this global pandemic. Our decades of experience confirm that it will be impossible to bring 30–40 million new CHB patients under treatment with commercially available NAs and PEG-IFN in order to achieve the WHO and SDG goal by 2030. NASVAC may well be the missing drug that is of a profound safety and efficacy profile and of finite duration of administration.

Orcid

Mamun Al Mahtab https://orcid.org/0000-0003-3728-3879

Md Abdur Rahim https://orcid.org/0009-0006-1431-1143

Noor-E-Alam Mohammad https://orcid.org/0009-0004-0629-9637

Ahmed L Moben https://orcid.org/0000-0002-4862-7692

Rokshana Begum https://orcid.org/0009-0003-3982-0970

Musarrat Mahtab https://orcid.org/0009-0001-7344-4812

Sheikh MF Akbar https://orcid.org/0000-0003-4537-3313