Mycobacterium w (Mw), also known as Mycobacterium indicus pranii (MIP), is a highly sought-after leprosy vaccine initially produced domestically in India (45). Initially characterized as a leprosy vaccine, genome screening revealed that MIP shares a vast repertoire of highly antigenic PE/PPE proteins with Mtb (46), indicating its potency as a TB vaccine candidate. While MIP administered subcutaneously diminishes the bacterial burden in the lungs, nasal administration of MIP further reduces the l burden and improves pulmonary pathology in mice (47). MIP induces autophagy, reversing phagosome maturation block and phagolysosome fusion inhibition, leading to enhanced Mtb clearance (48). TNF-α, IL-12p40, IL-6, nitric oxide, increased ROS, and IFN-induced chemokines like CXCL10 are among the pro-inflammatory reactions brought on by MIP (49). It triggers a T_H1 and T_H17 immune response generating IFN-γ⁺ TNF-α⁺ IL-2⁺ T cells, or IFN-γ⁺ TNF-α⁺ T cells (50), downregulates the T_H2 pathway, and stimulates dendritic cells and macrophages (51).

As a potential TB vaccine, MIP is presently undergoing Phase III clinical studies. These studies are examining its effectiveness and safety in preventing TB, especially in healthy household contacts of TB patients. MIP was employed as a prophylactic vaccine against leprosy in a double-blind clinical experiment in household contacts of leprosy patients. A retrospective review conducted 13 years later showed that MIP provided considerable protection against TB as well, in an area where TB is very endemic and the subgroup that had BCG as a child had a much lower incidence of TB (52). The Ministry of Science and Technology, Government of India, and Cadila Pharmaceuticals Ltd., India, financed a phase III randomized, double-blind, placebo-controlled, multicenter clinical trial (NCT00265226) to assess the safety and effectiveness of MIP as a treatment for category II TB patients in India. Following four weeks of treatment, the MIP group’s sputum culture conversion rate (67.1%) was noticeably greater than the placebo group’s (57%), suggesting that MIP can eradicate bacteria (15). These findings show that MIP is safe and may help TB patients eliminate Mtb and highlight MIP’s potential as a TB vaccine that may be used both therapeutically and preventively, calling for more research into its modes of action and the best ways to administer it.

SRL172, an inactivated whole cell BCG booster made from a heat-inactivated non-tuberculous mycobacterium that was deposited at the National Collection of Type Cultures (NCTC, London, UK) under accession number 11659, was used by the Dartmouth group to start their investigations.

In Argentina, newly diagnosed PTB patients received triple-dose immunotherapy using SRL172 in conjunction with short-course, directly monitored chemotherapy. Compared to placebo receivers, immunotherapy recipients had a quicker recovery to normal values in all immunological markers (53). SRL172 has finished a Phase 3 clinical investigation demonstrating immunogenicity and efficacy in preventing culture-confirmed TB when tested on HIV-positive people in Tanzania (NCT02063555) (16). These features make SRL172 potent for TB vaccine development. To completely clarify the possible involvement of SRL172 in international TB prevention programs, more studies should concentrate on evaluating efficacy in various populations, examining long-term protection, and improving dose regimens.

One of the few vaccines being developed exclusively as a therapeutic TB vaccine is the RUTI^® vaccine, which is presently in Phase III clinical trial. RUTI^® is a drug substance (DS) liposomal suspension that contains sucrose as a charge excipient and pure cellular fragments of Mtb bacilli grown under stress to simulate intra-granulomatous conditions. This confers the advantage of protecting against latent TB (54). Therefore, in experimental animal models of LTBI, the DS contains several antigens traditionally characterized as typical of replicating and non-replicating Mtb. RUTI^® causes a mixed T_H1/T_H2/T_H3 (T_REG) multi-antigenic immune response without causing systemic or local harm in animal models of TB (54) Compared to standard-of-care (SOC) chemotherapy, RUTI^® maintains the cellular immune response against ESAT-6 and dramatically lowers the bacterial burden in the lungs (55).

Phase I clinical trial (NCT00546273) in 2007 demonstrated the safety and immunogenicity of RUTI^® in healthy volunteers, showing that volunteers can develop specific humoral and cell-mediated immune responses to the RUTI^® vaccine (17). A phase II clinical trial of the RUTI^® vaccine (NCT01136161) was initiated in three South African regions in HIV-positive and negative LTBI individuals. The findings showed that while HIV-positive patients exhibited a similar poly antigenic immune response profile following the first vaccination, no significant increase in immune response was observed following the second vaccination. In comparison, HIV-negative patients who received RUTI^® showed good immune responses and a slight increase in immune response intensity after the second vaccination (18). These findings imply that the RUTI^® vaccine-induced immune responses vary across HIV-negative and positive populations, maybe as a result of the compromised CD4⁺ T cells in HIV-positive people. Two-phase IIb clinical trials (NCT04919239 and NCT05455112) are now seeking volunteers to assess the safety and efficacy of RUTI^® immunotherapy in TB patients in comparison to conventional treatments, as well as the efficacy of RUTI^® as an adjuvant for TB chemotherapy.

The clinical strain Mtb Mt103 is a member of lineage 4 (Euro–African–American), one of the most prevalent lineages of Mtb, and contains most T cell epitopes specific to TB (56). MTBVAC is a live vaccine candidate rationally attenuated from this lineage. According to the Geneva Consensus guidelines for creating live-attenuated mycobacterial vaccines, MTBVAC attenuation is provided by two separate, unmarked deletions in the phoP and fadD26 pathogenicity genes. Studies on mice, guinea pigs, and rhesus macaques have shown that MTBVAC is safe, protective, and immunogenic (57, 58). Furthermore, co-administration of MTBVAC and BCG provides better protection in guinea pigs and adult and neonatal mouse models (59, 60).

MTBVAC reached clinical evaluation in newborns (NCT02729571) and adolescents (NCT02933281) in TB-endemic countries after completing the first-in-human phase 1 clinical trial for safety and immunogenicity in healthy adult volunteers in Switzerland in 2015 (19). Intradermal MTBVAC immunization has been shown to be acceptable reactogenicity and induced a durable CD4⁺ T-cell response similar to BCG in early-stage clinical studies in both adults and newborns (20). Two clinical trials, NCT02933281 and NCT03536117, are presently being conducted in South Africa to examine the many facets of MTBVAC. Additionally, MTBVAC (NCT04975178) is undergoing a randomized, double-blind, controlled phase III clinical trial in sub-Saharan Africa’s TB-endemic regions to assess its immunogenicity, safety, and effectiveness in infants with and without HIV exposure.

Giving BCG again as a homologous boost is a clear way to increase its effectiveness. This approach is crucial because, should BCG revaccination enhance the protective efficacy of this century-old vaccine, we would have a low-cost, easily implementable instrument to aid in the improvement of TB control on a worldwide scale. The BCG vaccine has wide immunomodulatory effects on the innate and adaptive immune systems in addition to providing targeted protection against TB. In high-mortality areas, the BCG vaccine lowers infant mortality by over 40% and is used to treat malignancies including melanoma and bladder cancer (61, 62).

The current BCG vaccination schedule for infants is insufficient due to the lack of effective TB vaccine candidates. 78 nations, including China, advised giving the vaccine twice or more at varied intervals before 2019 (63). An alternate strategy using BCG prime-heterologous boosting regimens has emerged, but preclinical investigations have not shown enhanced efficacy against primary Mtb infection (64). Due to increased IL-2⁺ T_CM and IFN-γ⁺ T_EM cell responses after the booster vaccination, BCG-primed mice and then either revaccinated with BCG (65) or heterologous boosted with subunit protein CMFO vaccine are effective methods for preventing and controlling LTBI rather than primary Mtb infection (66). A study investigated the suboptimal efficacy of the BCG prime-boosting strategy against primary Mtb infection in C57BL/6J mice. Results showed that IL-10 is upregulated in the lungs of BCG-revaccinated mice, and blocking IL-10 signaling increases antigen-specific IFN-γ⁺ or IL-2⁺ CD4⁺ T cell responses and better protection against aerosol infection (67). BCG revaccination effectiveness against primary Mtb infection is less than 50%, with only individuals aged 31–40 showing protective effects. This necessitates more investigation into the ideal time frames between BCG vaccinations (68).

Phase 2b clinical trials are presently being conducted on BCG revaccination as a potential TB vaccine. A placebo-controlled trial of repeat BCG immunization in Malawi showed no significant protection against TB (21). A phase 2b trial found that BCG revaccination did not provide protection against sustained Mtb infection in HIV-negative adolescents (22). BCG revaccination has been employed in combination with other vaccine candidates which will be discussed in further sections.

A systematic review was conducted by Lawrence et al. to determine the effectiveness of BCG revaccination as a TB preventive measure. Studies and demographics differed in the protective efficacy of BCG revaccination; some showed only a moderate defense against the development of TB disease, especially in high-risk groups like healthcare professionals (69). A study conducted by Paulo dos Santos and colleagues found that neither the initial nor sustained QFT Plus conversion was shown to be protected by BCG revaccination; conversion was seen in 34 (3·4%) in the BCG group and 32 (3·2%) in the placebo group (23). Pre-sensitization with environmental mycobacteria, variations in BCG strain, administration site, and geographic location are all potential reasons for the disparities in protection offered by a single BCG therapy.

The Max Planck Institute for Infectious Biology created the recombinant BCG vaccination VPM1002 (rBCG ΔUreC:: hly), which improves BCG immunogenicity by substituting the urease C encoding gene (UreC) with the Listeriolysin O (LLO)-producing gene of Listeria monocytogenes (Hly) (71). As a result of this alteration, LLO is secreted, facilitating the entry of DNA and antigens into the cytoplasm of the host cell for the MHC processing, triggering autophagy, inflammasome activation, and apoptosis. Early gene expression analysis in mice following VPM1002 immunization showed that similar to THP-1 cells, IL-18 and IL-1β expression was elevated, as was the expression of IFN-inducible genes like Tmem173 (STING), Gbps, and other GTPases (72). By day 14, the lymph nodes of mice immunized with VPM1002 showed higher levels of apoptosis than those of mice immunized with BCG (72). Since it takes 12–14 days for T cells to reach the lungs after Mtb infection, the lungs of the mice immunized with VPM1002 had more IL-2⁺TNF⁺ double cytokine generating cells seven days after Mtb challenge than animals that had BCG vaccination suggesting recall response (73). More T_CM and T_FH cells, higher T_H17 responses, faster T cell migration to the lungs after Mtb challenge, and higher levels of anti-mycobacterial antibodies were all linked to the enhanced protection provided by VPM1002 immunization in the mouse model (72).

Phase III clinical studies are underway for VPM1002. To assess the safety and immunogenicity of VPM1002 in male human volunteers, a randomized, control, dose-escalating phase I clinical trial was conducted in Germany (NCT00749034). The findings showed that VPM1002 produces specific and dose-dependent immune responses (74). A follow-up phase II clinical trial (NCT01479972) was carried out in high-burden settings in South Africa in neonates who had not been exposed to HIV and had not received the BCG vaccination. After a single dose, the results showed that VPM1002 is safe, immunogenic, and immunologically tolerated in neonates (26). In a double-blind, randomized, active-controlled phase 2 trial (NCT02391415) in four South African health centers, the safety and immunogenicity of VPM1002 with BCG in newborns with and without HIV were assessed. Despite the immunogenic qualities of both vaccinations, VPM1002 produces fewer immune responses than BCG (27).

The small sample size of the aforementioned clinical studies is one of its limitations, making it challenging to create statistical comparisons of the immune responses induced by the two vaccines. Sample sizes have been raised to 2000 and 6940 participants in two recent phase III clinical trials (NCT03152903 and NCT04351685) that are still awaiting volunteer recruitment. These clinical trials aim to assess the safety and efficacy of VPM1002 in preventing TB infection in neonates and preventing TB recurrence in patients who have received successful treatment for TB.

GlaxoSmithKline (GSK) in the UK created the subunit candidate TB vaccine M72/AS01E, which consists of the adjuvant AS01E and the highly immunogenic Mtb proteins Mtb39A and Mtb32A. M72/AS01E produced T_H1-expressing, polyfunctional, M72-specific CD4⁺ and CD8⁺ T cells in mice (75). More fundamental research is required to fully comprehend the immune protection mechanisms of the M72/AS01E vaccination. Despite limitations related to the available animal studies, nonclinical evaluations (antigen-selection approach and in vivo preclinical data) and clinical safety and immunogenicity evidence, based on the capacity of the candidate vaccine responses, supported a proof-of-concept human trial (76).

The M72/AS01E TB vaccine candidate is currently in Phase 3 clinical trials, having been evaluated in a Phase 2b study. In individuals receiving treatment for TB, M72/AS01E produced strong M72-specific humoral and polyfunctional CD4⁺ T-cell mediated immune responses (28). The TB-treatment (adults receiving treatment for TB disease who have completed the intensive phase of treatment) and TB-treated (adults with a history of successfully treated pulmonary TB disease) groups had higher levels of polyfunctional M72-specific CD4⁺ T cells expressing CD40L⁺ TNF-α⁺ IFN-γ⁺, CD40L⁺ IL-2⁺ TNF-α⁺ IFN-γ⁺, or CD40L⁺ IFN-γ⁺ profiles than the TB-naïve (adults who have never had TB) group (28). HIV-positive and HIV-negative patients showed immunogenicity to M72/AS01E, with reactions persisting one year after vaccination. Polyfunctional CD4+ T-cell responses were produced seven days after administration (29). In a randomized, double-blind, placebo-controlled phase II clinical trial (NCT01755598) in Kenya, South Africa, and Zambia, the vaccine showed a 54% protective efficiency against Mtb infection in adults (30). The M72/AS01E immunization offered at least three years of immunological protection (49.7% protective efficacy) to prevent latent infections from becoming active TB cases (31). These studies show that M72/AS01E is immunogenic and safe for adults and adolescents.

Compared to a placebo, the two-dose M72/AS01E immunization increased seroconversion rates, according to a meta-analysis (77). M72/AS01E immunization produced strong, polyfunctional CD4+ T cells for three years, with TEM cells present and more memory TH1 cytokine-expressing responses than other novel vaccines (78). To assess the safety and immunogenicity of M72/AS01E in HIV-positive individuals receiving virus suppression and antiretroviral therapy, a randomized, placebo-controlled phase III clinical trial (NCT04556981) is presently underway in South Africa.

The recombinant subunit candidate vaccine GamTBvac was created and studied in response to the pressing need for TB vaccines around the world as well as to address the TB situation in Russia. GamTBvac is a recombinant vaccine that contains three Mtb antigens: Ag85a, CFP10, and ESAT6. These antigens are fused into two chimeric proteins from Leuconostoc mesenteroides that have a dextran-binding domain (DBD). These fusions were created using an adjuvant that contained CpG oligodeoxynucleotides (ODN), diethylaminoethyl (DEAE)-dextran 500 kDa, and dextran 500 kDa (79). GamTBvac showed substantial immunogenicity and notable protective benefits against the H37Rv strain in mice and guinea pigs TB models (80).

GamTBvac’s safety and immunogenicity in healthy persons who have received a BCG vaccination were assessed in a phase I clinical trial conducted in Russia. The half-dose vaccine group demonstrated the maximum immunogenicity (32). After that, 180 healthy volunteers who had not contracted Mtb and had received BCG vaccination participated in a double-blind, randomized, multicenter, placebo-controlled phase II clinical trial (NCT03878004). The vaccination produced substantial antigen-specific IFN-γ, T_H1 cytokines, and IgG antibodies (33). A phase III clinical trial (NCT04975737) has been started to evaluate GamTBvac’s safety and effectiveness in preventing PTB in HIV-uninfected people between the ages of 18 and 45 in light of these encouraging findings.

An immunological adjuvant known as IC31^® and the H4 antigen, a fusion protein made from two Mtb-antigens, Ag85B and TB10.4, make up the investigational vaccination H4:IC31 (AERAS-404). The rationale of the H4:IC31 vaccine is that by presenting Mtb-specific antigens in this context, T-cell immunity triggered by BCG may be enhanced, improving protection against TB. The leucine-rich peptide KLK and the synthetic oligonucleotide ODN1a combine to form the exclusive IC31 adjuvant (Valneva, Vienna, Austria).

A phase I trial of H4:IC31 in South African adults found it safe and immunogenic, with the 15 μg dose causing the most optimal immune response (34) In Switzerland, TB-negative, HIV-negative, BCG-unvaccinated adults (NCT02420444), TB-negative, HIV-negative, BCG-vaccinated adults (NCT02109874), HIV-negative, BCG-vaccinated adults in Sweden (NCT02066428) and Finland (NCT02074956), and HIV-uninfected, HIV-unexposed, BCG-primed infants in South Africa (NCT01861730) have all participated in several phase I clinical trials. Similar outcomes from these clinical trials have shown that H4:IC31 is safe for humans and triggers multifunctional CD4⁺ T_H1 responses and IFN-γ production (35). A 2014 clinical trial in South Africa assessed the safety, immunogenicity, and prevention of Mtb infection with H4:IC31 and BCG revaccination in HIV-uninfected adolescents. Both vaccines were immunogenic, but H4:IC31 users had a poorer conversion rate (24). In Cape Town, South Africa, a phase 1b randomized clinical study (NCT02378207) was carried out in 2020 to assess the immunological responses and safety of three distinct vaccination regimens: BCG revaccination, H4:IC31, and H56:IC31. The data showed that the H4:IC31 and H56:IC31 vaccines produced Ag85B-specific CD4⁺ T cell-mediated cellular immunological responses and H4 and H56 antigen-specific IgG antibodies (25). Presently, H4:IC31 is in Phase II clinical trials assessing safety and immunogenicity in infants with BCG vaccination and ongoing in Africa.

Glucopyranosyl lipid adjuvant (GLA)-stable emulsion (SE) is a synthetic TLR 4 agonist that was created as an adjuvant with an oil-in-water emulsion; ID93 is made up of four Mtb antigens (Rv2608, Rv3619, Rv3620, and Rv1813) linked to virulence or latency (81). The production of T_H1 CD4⁺ T cells by ID93/GLA-SE results in long-lasting immunity and inhibits pulmonary disease in C57BL/6 mice both early (4 weeks) and late (8 months) after challenge with Mtb HN878 (82). Several animal models have already shown the safety, immunogenicity, and pharmacodynamics of this vaccine (83).

To assess the safety and immunogenicity of two doses of ID93 antigen, either by itself or in adjunct with two doses of GLA-SE adjuvant, in healthy people, a phase I randomized, double-blind, dose-escalation clinical trial (NCT01599897) was carried out in the United States. The findings demonstrated that ID93, in adjunct with adjuvant GLA-SE, produced potent antibody and CD4⁺ T cell immunogenic responses (36). Similar results were obtained in follow-up phase I randomized, controlled, placebo-controlled, dose-escalation experiment (NCT01927159) carried out in South Africa in healthy HIV-negative people who have received the BCG vaccination (37) as well as in a phase IIa randomized, double-blind, placebo-controlled clinical trial (NCT02465216) conducted in Cape Town, South Africa, in adult TB patients who were HIV-uninfected following treatment (38). Furthermore, ID93+GLA-SE also made strong, long-lasting antibody responses and antigen-specific, multifunctional CD4⁺ T cell responses (38). A phase 2a trial in South Korea found that administering the ID93+GLA-SE vaccine to HIV-negative, BCG-vaccinated, and QuantiFERON-TB-negative adults significantly increased antigen-specific antibody levels and T_H1 immune responses (39). All clinical trials of ID93+GLA-SE in various groups showed good safety and immunogenicity, which strongly supports the need for additional clinical trials. However, statistical differences cannot be reliably determined due to the small sample sizes in these investigations.

Comprising the recombinant Mtb antigen Ag85b, ESAT6-CFP10, and a complex adjuvant system BC02, AEC/BC02 is a novel recombinant TB vaccine that has successfully finished phase I clinical trials (84). Combining three or six injections of the AEC/BC02 vaccine with four weeks of isoniazid-rifampin tablet treatment could improve the pathological lesions and drastically lower the bacterial load and gross pathological score in organs of a guinea pig latent infection model (85). These findings imply that AEC/BC02 might function as a therapeutic vaccination and stop latent infections from reactivating. AEC/BC02 vaccine immunotherapy significantly reduced bacterial loads in the lungs and spleen of mice, possibly related to the antigen-specific IFN-γ and IL-2 cellular immune responses induced by AEC/BC02 (86). This study also confirmed the therapeutic effect of the AEC/BC02 vaccine on latent infection with Mtb in mice.

Consequently, Anhui Zhifei Longcom Biopharmaceutical Co., Ltd. launched a phase I clinical research (NCT03026972) in 2017 to assess the safety of the AEC/BC02 vaccine. Additionally, a single-center, single-dose, placebo-controlled Phase Ib clinical trial (NCT04239313) was subsequently carried out in 2020 in healthy individuals. Although participant enrollment for both clinical trials is complete, the results have not been published. To assess the safety, tolerability, and immunogenicity of the AEC/BC02 vaccine in LTBI patients aged 18 and older, a phase II double-blind, randomized, controlled clinical trial (NCT05284812) was started in Hunan, China, in March 2022. Volunteers for the trial are currently being recruited. This study will offer more clinical proof of the effectiveness of AEC/BC02 as a possible TB vaccine.

MVA85A, the most clinically progressed novel vaccine candidate, is a recombinant replication-deficient MVA that expresses the immunodominant Mtb Ag85A. The BCG-MVA85A vaccination program can protect against Mtb in guinea pigs (88), NHPs (89), and cattle (90). It also produces high cellular immunity in mice and calves (90, 91).

Phase II clinical studies are presently being conducted on MVA85A. A phase I clinical trial (NCT00460590) was carried out in Cape Town, South Africa, in 2007 to assess the immunogenicity and safety of MVA85A in healthy participants. The findings demonstrated that the vaccination produced strong, long-lasting, and antigen-specific CD4⁺ T cell responses (92). A phase II clinical trial (NCT00679159) was carried out in 2014 in healthy infants and children who had received the BCG vaccination in South Africa. The findings, which were released seven years later, indicated that the MVA85A vaccination produced strong, long-lasting, polyfunctional CD4⁺ and CD8⁺ T cell responses in newborns (93). A randomized trial compared MVA85A prime vaccination to Candin^® control and BCG vaccination for HIV-uninfected infants. MVA85A vaccination induced higher Ag85A-specific IFN-γ⁺ CD4⁺ T cells than control, but BCG did not further boost this response. BCG-induced Ag85A-specific IFN-γ⁺ CD4⁺ T-cell response was similar across study arms (68). Although the precise mechanism behind MVA85A’s protective efficacy is unknown, its low antigen complexity is thought to be the reason for its clinical failure, which led to the development of many vaccine candidates with diverse antigens.

Live, non-replicating viral vector-based vaccines are safe and efficient (94). A new chimpanzee adenoviral vector vaccination that expresses Ag85A is ChAdOx1.85A. Both T cell and B cell responses were elicited by a combination of viral-vectored vaccines that expressed the immunogenic and immunodominant secretory TB Ag85a (ChAdOx1 85A prime, followed by MVA85A boost [ChAdOx1 85A–MVA85A]). In a mouse model, this combination of vaccines was more effective than BCG vaccination alone (95).

In healthy people in the UK, a Phase I clinical trial assessed the safety and immunogenicity of the ChAdOx1.85A prime-MVA85A boost vaccine, showing good immunogenicity and tolerance (40). The proposed TB vaccine ChAdOx1-85A was administered by aerosol versus intramuscular route in a randomized, double-blind, controlled phase 1 experiment (SNCTP000002920). The intramuscular ChAdOx1-85A vaccination produced stronger humoral and systemic cellular responses, while the aerosol ChAdOx1-85A vaccination produced stronger BAL cellular responses, especially IFN-γ/IL17⁺CD4⁺ T cells (41). The viral vector intramuscular vaccination ChAdOx1.85A + MVA85A finished its phase IIa clinical study in 60 adolescents. Compared to the BCG vaccine, the vaccine produced greater Ag85A-specific responses (42).