Peptide-Based COVID Vaccine Candidates
Peptide Vaccines for Infectious Disease
3 Candidates
Three peptide-based COVID-19 vaccines reached clinical trials, each using a fundamentally different approach from the mRNA vaccines that dominated the pandemic response.
Heitmann et al., Nature, 2022
Heitmann et al., Nature, 2022
View as imageWhen COVID-19 triggered the fastest vaccine development program in history, mRNA vaccines from Pfizer-BioNTech and Moderna captured most of the attention. Viral vector vaccines from AstraZeneca and Johnson & Johnson took the remaining spotlight. But a third approach, quieter and slower, was running in parallel: peptide-based vaccines that use short synthetic protein fragments to train the immune system against SARS-CoV-2. By mid-2021, over 350 interventional clinical trials involving peptide-based vaccines were registered on ClinicalTrials.gov, though the vast majority were in early stages and only a handful specifically targeted SARS-CoV-2.
Three peptide COVID vaccine candidates reached human clinical trials. CoVac-1, developed at the University of Tubingen in Germany, focused exclusively on inducing T-cell immunity using carefully selected viral epitopes.[1] EpiVacCorona, developed at Russia's VECTOR institute, claimed 82.5% efficacy in a Phase III trial but faced serious scientific scrutiny.[2] UB-612, from Vaxxinity (formerly COVAXX), combined a recombinant protein with synthetic peptide epitopes in a multitope design that completed a Phase III booster trial. Each took a fundamentally different path, and each revealed something about the strengths and limitations of peptide-based vaccines in a pandemic.
The broader context of how peptide-based vaccines work for infectious disease provides the mechanistic foundation for understanding these COVID-specific candidates. This article focuses on what happened when those principles met a real-world pandemic.
Key Takeaways
- CoVac-1 induced SARS-CoV-2-specific T-cell responses in 100% of 36 healthy volunteers, with responses surpassing those from infection and approved vaccines (Heitmann et al., Nature, 2022)
- In a Phase I/II trial, CoVac-1 induced T-cell responses in 86% of 54 immunocompromised patients with B-cell deficiency, a population poorly served by antibody-based vaccines (Heitmann et al., Nature Communications, 2023)
- EpiVacCorona reported 82.5% efficacy in its Phase III trial, but independent researchers found that conventional antibody tests could not detect post-vaccination antibodies, raising questions about the claimed protection
- UB-612 met non-inferiority as a heterologous booster against homologous boosters in a Phase III trial across multiple countries
- All three peptide COVID vaccines demonstrated favorable safety profiles with no serious vaccine-related adverse events
- No peptide-based COVID vaccine achieved widespread deployment; the speed advantage of mRNA platforms proved decisive during the pandemic
How Peptide Vaccines Differ From mRNA Vaccines
The mRNA vaccines that dominated the pandemic response work by delivering genetic instructions for the full SARS-CoV-2 spike protein. The cell reads those instructions, builds the spike protein, and the immune system learns to recognize it. This approach primarily generates neutralizing antibodies against the spike, which block viral entry.
Peptide vaccines take a different approach. Instead of encoding an entire protein, they deliver pre-selected short fragments, typically 8-30 amino acids long, that represent the most immunologically important parts of the virus. These fragments, called epitopes, are chosen because they bind to human leukocyte antigen (HLA) molecules on cell surfaces, triggering T-cell recognition.[3]
The distinction matters in practice. mRNA vaccines excel at generating antibodies but those antibodies lose potency as the virus mutates, which is why COVID boosters became necessary with each new variant. Peptide vaccines, particularly those designed around T-cell epitopes, can target conserved regions of the virus that mutate less frequently. T-cell immunity also provides a second layer of defense: even when antibodies fail to prevent infection, T cells can kill infected cells and limit disease severity.
The trade-off is speed and scalability. mRNA vaccines can be redesigned in days and manufactured at massive scale. Peptide vaccines require careful epitope selection, often guided by immunopeptidomic analysis of which viral fragments are naturally presented on human cells.[4] This precision takes time. In a pandemic, time is the scarcest resource.
There is also a manufacturing consideration. Synthetic peptides are produced by solid-phase peptide synthesis (SPPS), a chemical process that builds peptide chains one amino acid at a time. This process is well-established, reproducible, and does not require cell culture or biological systems. A peptide vaccine can be manufactured in a chemical facility rather than a biologic manufacturing plant, reducing infrastructure requirements. Once the epitope sequences are determined, production can be scaled to any facility capable of SPPS, which includes hundreds of contract manufacturers globally.
Computational tools for predicting which peptide fragments will bind HLA molecules and activate T cells have advanced substantially, but prediction accuracy remains imperfect.[5] The best peptide vaccine designs combine computational prediction with experimental validation through mass spectrometry-based immunopeptidomics, confirming that the selected peptides are actually presented on human cells under real conditions. The gap between computational prediction and biological reality is where vaccines like EpiVacCorona appear to have failed: the selected peptides may have looked promising in silico without adequate validation that they trigger protective immune responses in vivo.
CoVac-1: The T-Cell Vaccine
CoVac-1 was developed by a team at the University of Tubingen led by Juliane Walz and Helmut Salih, building on their expertise in cancer peptide immunotherapy. The vaccine contains SARS-CoV-2 T-cell epitopes derived from multiple viral proteins, not just the spike, combined with the Toll-like receptor 1/2 agonist XS15 emulsified in Montanide ISA51 VG as an adjuvant. It is administered as a single subcutaneous injection.
The Phase I open-label trial enrolled 36 healthy participants aged 18-80 years. The primary endpoint was safety through day 56, with T-cell response as the main secondary endpoint. No serious adverse events and no grade 4 events were observed. Local granuloma formation at the injection site occurred in all participants, a known effect of the Montanide adjuvant that creates a depot effect for sustained antigen release. Systemic reactogenicity was absent or mild.[1]
The immunogenicity results were striking. SARS-CoV-2-specific T-cell responses targeting multiple vaccine peptides were induced in all 36 participants (100% response rate), mediated by multifunctional T-helper 1 CD4+ and CD8+ T cells. These CoVac-1-induced IFNgamma T-cell responses persisted at three-month follow-up and surpassed those detected after natural SARS-CoV-2 infection as well as after vaccination with approved mRNA and viral vector vaccines. The T-cell responses were unaffected by variants of concern circulating at the time, including Delta and Omicron.
The real test came in a Phase I/II trial in immunocompromised patients. Fifty-four patients with congenital or acquired B-cell deficiency, 50 of whom had cancer (leukemia or lymphoma), received a single CoVac-1 dose. These patients cannot generate adequate antibody responses to conventional vaccines, leaving them vulnerable to severe COVID-19 despite repeated mRNA boosters. CoVac-1 induced T-cell responses in 86% of these severely immunocompromised patients, directed against multiple vaccine peptides and unaffected by Omicron variants.
This finding identified CoVac-1's potential niche. For patients whose immune systems cannot make antibodies, a vaccine that works through T cells alone could provide protection that mRNA vaccines cannot. The broader role of peptides in HIV vaccine research has explored similar T-cell-focused strategies for decades, and the CoVac-1 COVID data validated that approach in a clinical setting.
The CoVac-1 team's expertise in immunopeptidomics, the mass spectrometry-based identification of naturally presented peptide antigens, was central to their design approach. Their "warehouse" concept, pre-selecting and pre-manufacturing a library of validated epitopes that can be assembled into personalized or disease-specific vaccines, was originally developed for cancer immunotherapy.[4] The same group published a clinical trial evaluation of this warehouse approach in chronic lymphocytic leukemia, demonstrating that the framework transfers across disease contexts.
For SARS-CoV-2, the warehouse approach meant that epitopes were not computationally predicted and immediately used. Instead, the team identified which SARS-CoV-2 peptide fragments are naturally processed and presented on HLA molecules in infected human cells, verified that these fragments trigger T-cell responses in convalescent COVID patients, and then selected a panel of epitopes covering multiple viral proteins and multiple HLA types to maximize population coverage. This multi-step validation pipeline distinguishes CoVac-1 from vaccine candidates that relied on computational prediction alone.
CoVac-1 has not advanced to a Phase III efficacy trial for the general population, partly because the pandemic vaccine landscape shifted before the opportunity arose. Its development has focused on immunocompromised populations, where the unmet need is clearest and where CoVac-1's T-cell-only mechanism represents a genuine advantage rather than a limitation.
EpiVacCorona: The Controversial Candidate
EpiVacCorona was developed at Russia's State Research Center of Virology and Biotechnology VECTOR, a former Soviet bioweapons facility repurposed for civilian vaccine research. It was registered as the world's first peptide-based antiviral vaccine for mass immunization. The vaccine contains three short synthetic peptides conjugated to a carrier protein, formulated with aluminum hydroxide as adjuvant.
The Phase III trial, published in Vaccines in 2023, reported prophylactic efficacy of 82.5% (95% CI: 75.3-87.6%) against symptomatic COVID-19. Vaccine administration produced mild local reactions in up to 27% of cases and mild systemic reactions in up to 14%.[2]
The controversy began almost immediately after the vaccine's rollout. Multiple independent concerns emerged:
Antibody detection failure. Conventional commercially available antibody tests could not detect post-vaccination antibodies in EpiVacCorona recipients. The vaccine developers created their own proprietary test system to measure immune responses, but declined to reveal the specific antigens used. Critics argued that this test might detect antibodies to vaccine components (like the carrier protein maltose binding protein) rather than antibodies that neutralize SARS-CoV-2.
Trial participant reports. Russian citizens who participated in the trial publicly reported that commercial antibody tests showed no virus-neutralizing antibodies after vaccination. Letters to Russia's health minister called for independent review. Some trial participants reported contracting COVID-19 despite full vaccination.
Real-world effectiveness data. A test-negative case-control study in St. Petersburg comparing Russia's three COVID vaccines (Sputnik V, EpiVacCorona, and CoviVac) found that EpiVacCorona was the only vaccine that failed to show effectiveness against lung injury during both Delta and Omicron surges.
Methodological criticism. An international team of scientists published a detailed critique of the Phase III publication, identifying inconsistencies and ambiguities in the reported data that they argued undermined the 82.5% efficacy claim.
EpiVacCorona illustrates a risk inherent to peptide vaccine design: if the selected peptide epitopes do not match the actual immune targets needed for protection, a vaccine can generate immune responses that look positive on a customized assay without providing meaningful clinical protection. The vaccine was administered to millions of Russians before these concerns fully surfaced.
The fundamental scientific question that EpiVacCorona raises is whether short linear peptides (the three epitopes used were reportedly 20-30 amino acids long) can adopt the correct three-dimensional conformation needed for antibody recognition when removed from their native protein context. Antibodies typically recognize conformational epitopes, the shape of a folded protein surface, not the linear peptide sequence that encodes it. Short peptides in solution may not fold into the conformation the immune system needs to learn. This is a known limitation of first-generation peptide vaccine designs and is one reason that newer approaches like UB-612 combine folded protein domains (the RBD) with linear peptide epitopes designed for T-cell rather than antibody recognition.
The VECTOR institute's decision to develop its own antibody detection assay rather than validating immune responses with standard commercial tests remains the central credibility issue. In vaccine science, immune correlates of protection must be measurable by independent laboratories using validated assays. A vaccine whose immune responses can only be detected by the developer's proprietary test does not meet this standard.
UB-612: The Multitope Approach
UB-612, developed by Vaxxinity (originally COVAXX, a division of United Biomedical), took a hybrid approach that combined protein subunit and peptide technologies. The vaccine contains a recombinant S1 receptor-binding domain (RBD) protein fused to a single-chain Fc fragment for antibody generation, plus five rationally designed synthetic peptides representing conserved T-cell epitopes from the spike S2, nucleocapsid (N), and membrane (M) proteins.
This multitope design aimed to induce both neutralizing antibodies (via the RBD component) and broad T-cell immunity (via the peptide epitopes). The peptide epitopes were selected from conserved regions across sarbecoviruses, meaning amino acid sequences that remain nearly identical between SARS-CoV-1, SARS-CoV-2, and related bat coronaviruses. This conservation provides theoretical cross-protection against emerging variants and potentially future coronavirus spillovers from animal reservoirs.
Phase I/II trial results showed that UB-612 induced neutralizing antibody titers comparable to human convalescent sera, with a favorable safety profile. The vaccine showed strong T-cell responses directed at the nucleocapsid and membrane proteins, targets that mRNA vaccines (which encode only the spike protein) do not address.
The Phase III trial, conducted from March to September 2023 across multiple countries, evaluated UB-612 as a heterologous third-dose booster in adults who had received primary vaccination with mRNA, adenoviral vector, or inactivated virus vaccines. The trial met its primary non-inferiority endpoint versus homologous boosters for neutralizing antibody responses against the ancestral Wuhan strain. UB-612 outperformed ChAdOx1 (AstraZeneca) and Sinopharm (BBIBP) boosters on neutralizing antibody responses including against Omicron subvariants, and showed durability comparable to BNT162b2 (Pfizer) through 12 months.
Despite these results, UB-612 did not achieve widespread regulatory approval or deployment. The pandemic vaccine market had consolidated around mRNA platforms by the time UB-612's Phase III data matured, and variant-updated mRNA boosters had already captured most of the remaining demand.
UB-612's trajectory illustrates a structural challenge for alternative vaccine platforms during a pandemic. By the time a peptide or protein-based vaccine completes the clinical trial sequence (Phase I through Phase III), the mRNA vaccines have already been deployed to billions of people, the regulatory infrastructure has been optimized for mRNA platforms, and procurement contracts lock in supply chains. Breaking into a market already served by an established platform requires either dramatically superior efficacy (which UB-612 did not demonstrate) or a clear niche advantage.
The scientific contribution of UB-612, however, is real. By demonstrating that synthetic peptide epitopes from conserved non-spike proteins can augment antibody responses from a protein subunit component, the vaccine proved a design principle: hybrid peptide-protein vaccines can simultaneously induce both arms of adaptive immunity. This architecture could be applied to future pandemic preparedness vaccines where breadth of coverage matters as much as peak antibody titers.
What the Pandemic Revealed About Peptide Vaccines
The COVID-19 pandemic served as an unintentional stress test for peptide vaccine technology. Three conclusions emerged clearly:
Speed disadvantage is real. mRNA vaccines went from sequence to emergency authorization in under a year. Peptide vaccines require epitope selection, synthesis, formulation, and adjuvant optimization before clinical testing begins. CoVac-1's first-in-human dose was administered in November 2020, nearly a year after the SARS-CoV-2 sequence was published, while mRNA vaccines were already in Phase III trials by that point. This inherent timeline disadvantage is manageable for diseases with long development horizons but was decisive during a pandemic.
T-cell targeting fills a genuine gap. CoVac-1's results in B-cell-deficient patients demonstrated that peptide vaccines targeting T-cell immunity can protect populations that antibody-focused vaccines leave behind. This finding has implications beyond COVID for CMV protection in transplant patients and HPV peptide vaccine development, where T-cell responses are central to protection.
Epitope selection quality determines everything. EpiVacCorona's failures were not failures of peptide vaccines as a category. They were failures of epitope selection and validation. CoVac-1 used rigorous immunopeptidomic analysis to identify naturally presented epitopes. EpiVacCorona used short synthetic peptides without comparable validation. The technology platform is neutral; the science behind epitope selection makes or breaks the product.
Adjuvant formulation shapes the response. CoVac-1's use of the TLR-1/2 agonist XS15 in Montanide created a depot effect that sustained antigen presentation and drove potent T-cell responses from a single injection. EpiVacCorona used aluminum hydroxide, a standard adjuvant that primarily enhances antibody responses rather than T-cell immunity. UB-612 used CpG 1018 plus aluminum hydroxide, a combination designed to balance both arms. The choice of adjuvant is not an afterthought in peptide vaccine design; it determines the character of the immune response at least as much as the peptide antigens themselves. The peptide-TLR conjugate approach described by Lynn et al. takes this principle further by chemically linking adjuvant directly to the peptide antigen, ensuring co-delivery to the same antigen-presenting cell.[3]
The antiviral peptide field extends beyond vaccines into direct viral inhibition. Peptides like the pan-coronavirus fusion inhibitor EK1 block viral entry through a different mechanism entirely, targeting the spike protein's HR1 domain rather than training immune cells.[6] The body's own defensin peptides also directly inhibit SARS-CoV-2 by blocking viral entry, independent of adaptive immunity.[7]
The Future of Peptide COVID Vaccines
The question is no longer whether peptide vaccines can work against COVID-19. CoVac-1 proved they can induce potent, variant-independent T-cell immunity. The question is where they fit in a post-pandemic landscape dominated by established mRNA platforms.
Three niches appear most viable:
Immunocompromised populations. Patients with B-cell deficiency, transplant recipients on immunosuppressive drugs, and cancer patients undergoing B-cell-depleting therapies need vaccines that work through T cells. Peptide vaccines designed for this population could become standard of care if Phase III trials confirm CoVac-1's Phase I/II signals.
Pan-sarbecovirus preparedness. UB-612's use of conserved T-cell epitopes from non-spike proteins represents a preparedness strategy. A peptide vaccine pre-designed around epitopes conserved across all sarbecoviruses could provide baseline T-cell immunity against future pandemic coronaviruses before they emerge, a concept that resonates with pandemic preparedness planning. Pan-coronavirus fusion inhibitor peptides like EK1 have demonstrated that conserved viral targets can be exploited pharmacologically.[8]
Cold-chain-independent distribution. Synthetic peptides are chemically stable and can be manufactured, stored, and transported without the ultra-cold chain requirements of mRNA vaccines. This matters for global health equity in regions where -70C freezers are unavailable. The peptide-TLR conjugate approach demonstrated by Lynn et al., which programs peptide antigens to self-assemble into stable nanoparticles, could further simplify distribution logistics.[3]
None of these niches represents the mass-market scale of pandemic mRNA vaccination. But they represent real unmet needs where the specific properties of peptide vaccines, T-cell specificity, stability, variant independence, offer genuine advantages over the incumbent platforms.
The peptide vaccine field also benefits from ongoing advances in epitope prediction, adjuvant chemistry, and delivery technology. T-cell epitope prediction algorithms have improved substantially since the early pandemic, and mass spectrometry-based immunopeptidomics can now characterize virus-derived peptides presented on human cells within weeks rather than months. If the next pandemic coronavirus emerges, the time required to design and validate a peptide vaccine will be shorter than it was for SARS-CoV-2, though likely still slower than an mRNA vaccine update.
For peptide vaccines being developed against other viruses, the COVID experience provides concrete data points on what works (rigorous epitope validation, potent adjuvants, T-cell focus for immunocompromised populations) and what fails (inadequate validation, antibody-only endpoints for short linear peptides, proprietary immune assays).
The Bottom Line
Three peptide-based COVID-19 vaccines reached clinical trials with different approaches: CoVac-1 (T-cell focused, strong immunogenicity in healthy and immunocompromised patients), EpiVacCorona (disputed efficacy despite Phase III completion), and UB-612 (multitope design, positive Phase III booster data). The pandemic proved that peptide vaccines can induce potent T-cell immunity but cannot match mRNA platforms on speed. Their future lies in immunocompromised populations, pan-sarbecovirus preparedness, and distribution-stable formulations.