Live vaccine clinical trials bacteria

In recent published clinical studies, researchers tend to compare Nab levels between vaccines and convalescent COVID patients.

However, antibody response varies by time and between convalescent patients, with severe or older patients having higher antibody titers. In addition to humoral immunity, cellular immunity and local mucosal immunity play important roles in protection against SARS-CoV-2 infection as well. Mutations in spike protein, especially the RBD predicting conformational changes in the S1 domain, may compromise the efficacy of vaccines.

The granting of emergency use designation to candidate vaccines and licensed vaccines being available raise some issues. In some countries and regions with intensive COVID vaccination campaign, a randomized, double-blind placebo-controlled phase 3 efficacy clinical trial is tough to develop and maintain.

Investigators might be requested to unmask trial subjects to guarantee that those who received placebo are offered or actively seek approved candidate vaccines. Head-to-head comparative design, stepped-wedge design and cross-over design are suggested as alternative study design to avoid ethical issue as all people in the trial are offered protective vaccine, [ 78 , 79 ] but at a considerable cost to efficiency and benefit.

If possible, a serological correlate of protection and an immunological surrogate endpoint are expected to be justified by scientific evidence. At the present, vaccine efficacy results are just the relatively short-term data, and the durability of protection needs to be observed. Also, the number of severe cases observed in trials were still limited, in order to obtain a solid vaccine efficacy for severe cases, more severe COVID cases need to be captured in the continuing surveillance of phase 3 trials.

Furthermore, the protective efficacy data of the inactivated vaccines is mainly in 18 to 59 adults, and more data of other populations should be collected to support the vaccine efficacy. Since the emergency use authorization and conditional licensure are not full licensures, WHO suggested it is ethically applicable to continue blinded follow-up of placebo recipients in existing studies and to continue perform placebo-controlled trials in order to yield unbiased evidence for the next vaccine candidates.

Although the existing efficacy results of vaccine against SARS-CoV-2 show the full expectations to reduce the disease and economic burden resulted from COVID pandemic, we are still devoting to developing various kinds of vaccines in order to satisfy the demand of the whole world. Hu-Dachuan Jiang drafted of the manuscript. Peng Zhang contributed to the literature search. National Center for Biotechnology Information , U. Published online Apr Author information Article notes Copyright and License information Disclaimer.

Infect Dis Immun ;1 1 — Supplemental digital content is available for this article. Received Jan The work cannot be changed in any way or used commercially without permission from the journal. This article is made available via the PMC Open Access Subset for unrestricted re-use and analyses in any form or by any means with acknowledgement of the original source.

These permissions are granted for the duration of the COVID pandemic or until permissions are revoked in writing. Protein based vaccines Many of the vaccines in clinical use today fall into this category.

Gene based vaccines Gene based vaccines have the potential to elicit broad immune responses and are easier to achieve mass production compared with protein-based vaccines. Table 1 The profile of published clinical studies. Open in a separate window. Safety The accumulated safety data from clinical trials shows that candidate COVID vaccines from different platforms are generally safe and tolerable, but with distinct safety profiles.

Figure 1. Figure 2. Table 2 The efficacy results from published clinical studies. NA: Not available. Conflicts of Interest None. References [1] World Health Organization. Accessed February 6, Nat Rev Immunol ; 20 10 — Ann Intern Med ; 5 — What happens to people who get seriously ill?

Accessed October 15, Measuring underreporting and under-ascertainment in infectious disease datasets: a comparison of methods. BMC Public Health ; 14 BMC Med ; 18 1 Lancet ; — Lancet Infect Dis ; 21 1 :3—5. Nat Med ; 26 11 — Science ; — Cell ; 2 — Nature ; — Cryo-EM structure of the nCoV spike in the prefusion conformation. Accessed January 29, JAMA ; 12 — Recent advances in subunit vaccine carriers.

Vaccines Basel ; 4 2 Inactivated virus vaccines from chemistry to prophylaxis: merits, risks and challenges. Expert Rev Vaccines ; 11 6 — NIAID researchers will introduce the bacteria Roseomonas mucosa from healthy skin onto the skin of someone with atopic dermatitis to see if it helps to treat the disease. Some blood tests are required for enrollment.

The study evaluates the safety and effectiveness of R mucosa in patients ages years. All participants will self-administer R mucosa twice a month for four months and assessments will occur on a monthly basis. Participants will then be monitored for up to one year to evaluate long-term effectiveness and safety. There is no charge to participate in this research study.

All study-related medical care, including clinic visits and procedures, are provided free of charge. Modest financial compensation will be provided to study participants.

Travel expenses may be reimbursed. Visit ClinicalTrials. Email: prpl mail. Watch a series of short informational videos about participating in clinical trials.

These videos are intended to help potential participants understand how research works, what questions they should consider asking, and things to think about when deciding whether or not to participate in a study.

Skip to main content. Skip to Back. Search for Resources. Division of Intramural Research Labs. Research at Vaccine Research Center. Clinical Research. Infectious Diseases. Resources for Researchers. Managing Symptoms. Understanding Triggers. Autoimmune Diseases. Disease-Specific Research. Characterizing Disease. Researcher Resources. Dengue Fever. Shiga Toxin-Producing E. Researching Ebola in Africa. Photo Essay. Food Allergy. Research Approach. Treatment for Living with Food Allergy.

Fungal Diseases. Group A Streptococcal Infections. Vaccine Research. Types of Group A Strep. Vaccine Development. Basic Research. Leprosy Hansen's Disease. Lyme Disease. Antibiotic Treatment. Live vaccines have played a critical role from the beginning of vaccinology. Indeed, the very first vaccination experiment in the Western world was Jenner's inoculation of a boy with the milder cowpox virus to protect against the deadly smallpox. Although effective the technology has safety problems associated with the risk of reversion to a virulent organism and the possibility of causing disease in immune compromised individuals.

Within the last 20 years the concept of live vaccines gains renewed interest due to our increased immunological understanding and the availability of molecular techniques making the construction of safer live vaccines possible. This opens for the development of new live bacterial vaccines that can avoid the downsides of parenterally administered vaccine because it i mimics the route of entry of many pathogens and stimulate the mucosal immune response ii can be administered orally or nasally avoiding the risk associated with contaminated needles and need for a professional healthcare infra structure iii has a simple down stream processing.

Broadly, live bacterial vaccines can be classified as a self-limiting asymptomatic organism stimulating an immune response to one or more expressed antigens. Furthermore, live bacterial vaccines can be designed to induce an immune response to itself or to a carried heterologous antigen. A non-virulent or attenuated derivative of the pathogen is used to induce a response to the bacterium itself.

When used as a vaccine vehicle the bacterium expresses an antigen from another species. Most commonly, these vaccine vehicles are based on either attenuated pathogens or bacteria used in the food industry. Both classes of bacteria deliver the vaccine component to the immune system whereby immunization may benefit from the bacterium's intrinsic adjuvant. The vaccine component to be delivered can be either protein or DNA.

In addition, the vaccine component may be a classical antigen but may also be allergens or therapeutics. A recent development is the use of invasive bacteria for the delivery of plasmid DNA vaccines to mammalian cells obtaining in vivo synthesis of the plasmid-encoded antigen.

As such, the applications of live bacterial vaccines are extensive and has lead to more than published papers.

However, only very few of the promising candidates have survived the licensing process and become registered [ 1 ] illuminating the difficulty in developing a commercial live vaccine.

One typhoid vaccine Ty21a contains live attenuated Salmonella typhi and is administered orally either as a liquid or as acid resistant capsules. Both formulations require three doses within one week to give immunity. The other registered vaccine based on live bacteria is against cholera and is given orally as a single dose of attenuated Vibrio cholerae CVD HgR in liquid formulation.

The very few examples of live bacterial vaccines on the market may be due to lack of success in clinical trials. However, we believe that the safety of these vaccines is another issue. Indeed, prophylactic vaccines are given to healthy people and despite excellent safety record they remain targets of un-substantiated allegations by anti vaccine movements. Furthermore, future live vaccines will most likely be either targeted mutagenised or equipped with foreign antigens and therefore considered recombinant.

As such, they fall into the debate on releasing genetically modified organisms into nature. The feasibility of this new vaccine strategy will therefore in particular depend on considerations of safety issues. We believe that considering safety issues alongside the scientific consideration early in vaccine development will facilitate its public acceptance and its entrance to the market.

We therefore felt compelled to outline a review about live vaccines and their safety aspects. Lindberg [ 2 ] has excellently reviewed the history of live bacterial vaccines. The first use of a live bacterial vaccine was in Spain in and consisted of a subcutaneous injection of weakened Vibrio cholerae. This study was followed a few years later by field trials in India with a more efficacious V.

The first live oral V. Later the V. In a bivalent vaccine waspresented including two strains of V. However, later on problems with attenuation of strain CVD appeared [ 4 ]. The development of the other registered live bacterial vaccine began Hg-in the early s using various live attenuated S. One proposed strain was made streptomycin-dependent, but failed to be efficacious in freeze-dried formulation [ 5 ]. Furthermore, the strain was genetically unstable and reverted to virulence.

Another S. This strain was extensively evaluated in several field trials and has shown excellent safety record [ 6 ].

Later, other auxotrophic strains unable to synthesise essential compounds like aromatic amino acids were developed and tested on human volunteers with variable safety and immunogenicity results [ 7 - 10 ].

Attenuated live vaccines to prevent shigellosis have also been proposed. Both genetically engineered or selected mutants of Shigella have been tried but showed side effects in clinical trials and points to the need of additional attenuation without hampering immunogenicity [ 11 - 13 ]. Kotloff et al attenuated the guanine auxotrophic Shigella flexneri 2a further by deleting two genes encoding enterotoxins [ 14 ].

In a phase 1 trial this strain with inactivated enterotoxin genes was better tolerated but still immunogenic compared to the guanine auxotrophic strain that contain active entoroxins.

Recombinant Shigella has also been proposed as a vaccine vehicle [ 15 ]. Pathogenic Shigella has a virulence plasmid encoding proteins involved in thesecretion apparatus and proteins necessary for the entry process into human cells. This invasive capacity can be used to deliver plasmid DNA vaccines into mammalian cells [ 16 ]. Here, the delivered plasmid DNA encodes an antigen, which is expressed by the protein synthesis apparatus of the infected cells.

Diaminopimelate Shigella auxotrophs undergo lysis unless diaminopimelate is present in the growth media [ 16 ]. Human cells contain low amounts of diaminopimelate and upon entry the Shigella mutant lyse making the delivery of vaccine components more effective. In conclusion, the mimicry of natural infection makes attenuated bacteria effective. The ability to deliver vaccine components of different origins like e.

However, in spite of the efforts in constructing attenuated pathogens for use as bacterial vaccine vehicles none of them has reached the market yet. The potential of using lactic acid bacteria LAB for the delivery of vaccine components is less exploited than attenuated pathogens. Due to their safe status and the availability of genetic tools for recombinant gene expression LAB are attractive for use as vaccine vehicles. Furthermore, their non-pathogenic status circumvents the need to construct attenuated mutants.

However, LAB are non-invasive and the vaccine delivery to antigen presenting cells may be less effective than invasive bacteria. Geoffroy et al [ 21 ] used a green fluorescent protein to visualize the phagocytosis of Lactobacillus plantarum by macrophages in vitro and in mice.

Macrophages act as antigen presenting cells and this can explain a possible way to at least elicit a ClassII MHC receptor presentation of the antigen. Even though the transit time of Lactococcus lactis through the intestine is less than 24 h in mice [ 22 ], a potent immune response has been obtained with several antigens including tetanus toxin fragment C TTFC.

Surprisingly, a similar response was induced using dead or alive Lactococcus suggesting that in situ antigen synthesis is not essential [ 23 ]. A slightly better result was in the same study obtained with L.

The prospect of using live LAB as vaccine carriers has been reviewed [ 24 , 25 ]. The most frequently used model antigen is TTFC in which good results have been obtained both in intranasal and oral mice models using strains of L. Shaw et al [ 28 ] tested both cytoplasmic and surface associated expression of same TTFC antigen and found that cytoplasmic expression was superior to surface exposed TTFC in L. The different outcome of these experiments may be explained by different stability of surface exposed TTFC and E7 antigen.

Intracellular expression of a labile antigen can protect it from proteolytic degradation and environmental stress encountered at the mucosal surfaces. Genetic modification of the LAB cell wall rendering the strain more permeable increases the in vivo release of cytoplasmic TTFC antigen and was tested by Grangette et al [ 27 ].

When administered orally these alanin racemase mutants were more immunogenic than their wild type counterparts. One explanation could be that oral immunization is very dependant on a sufficiently large dose of the antigen [ 27 ]. The use of live LAB as carriers of DNA vaccines has until now not been an option as they are non-invasive and therefore inefficiently deliver the plasmid DNA to the cytoplasma of antigen presenting cells. This L. To determine the tropism of recombinant invasive strains Critchley-Thorne el al used a perfusion bath with murine ileal tissue and tested an invasive E.

Although change of tropism of a bacterial carrier opens for targeted delivery it introduces new safety issues that should be addressed by persistence and distribution studies of the bacterial strain after vaccination. Active vaccination using recombinant L. IgE epitopes was fused to proteinase PrtB and cell wall-anchored. Subcutaneous and intranasal immunization of mice induced a systemic IgG response against human IgE.

However, it remains to be proven if these antibodies are protective in human patients. Whether LAB will be effective as a mucosal vaccine in humans can only be answered by clinical trials. Furthermore, as the dose of recombinant LAB needed to elicit immune responses in animals is high it is unknown if the necessary dose for use in humans will be feasible and cost effective. Protection by preformed antibodies or antibody fragments is called passive vaccination.

The pioneer experiments were based on injection of antisera produced by immunized animals like horse or sheep to combat for example rattlesnake venom. Recently, passive immunity was delivered using lactobacilli that secretes single-chain antibodies [ 33 ]. In a rat caries model, colonisation of the mouth with a L. Recombinant Streptococcus gordonii displaying a microbiocidal single-chain antibody H6 has been used to treat vaginal candidiasis in a rat model [ 34 ].

Although passive immunity has limits in its temporary nature, these results suggest that LAB elegantly can be used for the delivery of neutralising antibodies at mucosal sites. For a normal vaccination against an infectious disease, induction of tolerance to the infectious agent is considered a side effect. This side effect is more prone to happen when vaccinating early in life [ 35 ].

However, induction of tolerance can have positive clinical implications when the purpose is to treat allergy. In a mouse model the use of a recombinant L.

In this study mice were sensitized by immunization with the house dust mite peptide and then given either L. But the decrease in production of-5 was only seen for the L. This indicates that the lactobacilli strain expressing Der p1 can suppress the cytokine milieu promoting the Th2 allergic response.

In this study L. The allergen can also be co-administered instead of recombinant expressed by the LAB. Mucosal co-application of L. Recombinant strains expressing immune polarizing cytokines like IL have also been developed and in vivo effects in both mice [ 39 ] and pigs [ 40 ] have been observed.

More knowledge on the mechanisms behind skewing the immune response is however needed to select the proper strain with anti allergic immune polarization. Furthermore, the immune regulatory effect of one strain of LAB may differ in allergic and non-allergic individuals.

A down regulation in allergic persons and an immune stimulating effect in normal persons was observed when using same strain of LAB [ 41 ]. Among LAB's effect on the immune system there is a strain dependent induction of cytokines.

Adding to the complexity of these observations, a human study has shown that non-specific immune modulation by a given strain of L. In healthy persons the strain was immune stimulatory whereas in allergic persons it down-regulated an inflammatory response [ 44 ]. Interactions between different LAB strains can also interfere with the in vitro production of cytokines by dendritic cells [ 45 ]. As is shown in another study [ 46 ], two different lactobacilli with similar probiotic properties in vitro were shown to elicit divergent patterns of colonisation and immune response in germfree mice.

Further evidence for an immune modulating effect is seen when either L. The immune polarizing effect of LAB has also been observed in humans. A clinical trial showed a strain dependent immune modulation of two different LAB strains when administered together with an oral S. Here, thirty healthy volunteers were randomised into three groups receiving L.

On days 1, 3 and 5 the Ty21a vaccine was given orally. Analysis showed a higher number of specific IgA-secreting cells in the group receiving L. A partial down regulation of the immune system has also been observed. It can be concluded that immune polarization towards either a Th2 or a Th1 response can be obtained using different LAB.

As such the intrinsic immune modulatory capacity of the LAB must be evaluated and selected to fit the purpose of vaccination. Before using pathogenic bacteria for vaccination purposes, its pathogenicity must be weakened via attenuation.

Attenuation usually involves deletion of essential virulence factors or mutation of genes encoding metabolic enzymes whose function is essential for survival outside the laboratory. Inactivation of a metabolic gene has the advantage that the bacteria still express virulence determinants important to elicit a protective immune response.

Appropriate stable auxotrophic strains are usually not able to replicate in the human body and can safely be used even in immune compromised individuals. Defined deletions of at least two metabolic essential genes are usually used [ 2 ] and decrease the probability of reversion to virulence.

To reduce the risk of spreading foreign genetic material to the environment the antigen encoding gene cassette can be inserted into the chromosome replacing the metabolic essential gene. If the bacterium acquires the deleted gene it will automatically loose the antigen-encoding cassette.

The use of antibiotic resistance genes as marker genes in vaccines is not encouraged as these genes can transfer to in the end humans and thus hamper the use of therapeutic antibiotics. Different alternatives to antibiotic resistance marker genes have been published and should be used as soon as possible in the developmental process of a vaccine [ 48 - 50 ].

Another concern using live bacterial vaccines is the onset of autoimmune responses like arthritis especially in patients with the HLA-B27 tissue type [ 51 ]. However, the risk is certainly lower than after natural infection. The occurrence of such side effects can best be followed by post launch monitoring and must always be evaluated against the health risks associated to the disease itself.