20 Feb 2023 Episode 318 Marburg infections and candidate vaccines

Mon, 02/20/2023 - 21:36

Episode 318: Marburg virus

Dear colleagues,

My attention was caught by a short paper by Ewen Callaway in Nature Briefing (Ep 318-1) on the first Marburg outbreak in Equatorial Guinea, starting in January with 9 deaths amongst 25 suspected cases as of Feb 13. WHO organized a meeting to decide if vaccine trials could be organized. Reportedly, there are a handful Marburg candidate vaccines, all based on viral vectors: either live vesicular stomatitis virus (VSV) or non-replicating chimp or human Adenoviruses.  All have shown protection in macaques, but there is little experience in humans and only the Janssen vaccine has a few thousand doses, produced in Beerse (Belgium).  But the question is whether it is feasible to organize a clinical vaccine trial as most of these outbreaks subside rapidly, once containment measures are in place.  Or should we accept animal evidence and “immune-bridging” to approve much-needed vaccines in exceptional circumstances? 

To investigate this question, I will provide a broad introduction, based on a very didactic chapter in a 2018 textbook on “Medical Aspects of Biological Warfare” by the US army and then dive into the limited specialized Marburg literature, while often  referring to and extrapolating from Ebola, because of the similarities and the more extensive knowledge on that virus.


Ep 318-2: Shelir Radoshitzky Chapter 23 Filoviruses p. 569

Virological aspects of filoviruses

Phylogenetic tree of mono-negavirales = negative strand RNA virus


Filovirus subfamilies:

2 Mar­burgviruses: Marburg virus (MARV) and Ravn virus (RAVV)

1 Cuevavirus: Lloviu virus (LLOV);

5 Ebolaviruses: Ebola virus (EBOV), Bundibugyo virus (BDBV), Taï Forest virus (TAFV), Sudan virus (SUDV), Reston virus (RESTV)


Filovirus genome organization = negative strand RNA virus (mono-negavirus)


All filovirus genomes have the same overall sequence of genes (rectangles) and open reading frames (horizontal arrows), but differ from each other in the number and position of gene overlaps (triangles) and intergenic regions.


ATP: adenosine triphosphate; IFN: interferon; IRF: interferon regulatory factor; JAK1: Janus kinase 1; MAPK: mitogen-activated protein kinase; MDA-5: melanoma differentiation-associated protein-5; MTAse: methyltransferase; NEDD4: neural precursor cell-expressed, developmentally down-regulated protein 4; NPC1: Niemann-Pick C1 protein; RIG-1: retinoic acid-inducible gene-1;

RNAi: RNA interference; RNP: ribonucleoprotein complex; Tsg101: tumor susceptibility gene 101 protein



Schematic picture of MARV (from Ep 318-3)


Top, the Marburg virus structure along with depicting the structural proteins. Bottom, an illustration of the genome organization of the Marburg virus. This seven-gene strain of Marburg virus has been drawn roughly to scale. The light blue boxes indicate noncoding areas, as well as the colored box code regions for genes. The red arrows demonstrate the position of the transcriptional start signals, whilst the pale brown bars highlight conserved transcriptional stop signals. The genes are segregated by intergenic regions, indicated using black arrows, with the exception of the overlapping sequence (black triangle) between VP24 and VP30. At the extreme ends, the 3' and 5' trailer sequence is shown.


Ebola and Marburg virus disease outbreaks


Most outbreaks are small, up to a few hundred cases. The exception is the 2013-2015 EBOV outbreak in West Africa.

The mortality of MARV 80 % > SUDV 53 % ≈ EBOV 46 % > BDBV 34 %  (RAVV and TAFV are very rare in humans).



Natural reservoir = still uncertain, but fruit bats (Egyptian rousettes) are highly suspected for MARV and RAVV


  • direct person-to-person contact: close contact with skin or mucous membranes, espe­cially in the presence of small lesions; genital, nasal, and other bodily secretions;
    • between sick people and their family members, friends, or health­care workers who care for them;
    • between deceased people and people who prepare bodies for funerals
  • also possible via contaminated (medical) material, aerosols during (medical) procedures: centrifugation of samples, in­tubation of patients, or suction used during surgical procedures.  

Clinical presentation 


These diseases are known as “hemorrhagic fever”, hence bleedings in various places, fever and malaise are common, but also headache as well as various gastro-intestinal and respiratory complaints.


Some promising vaccine candidates


Clearly, viral vector vaccines are most advanced:

  • Life vectors: mainly Vesicular Stomatitis Virus (VSV) and each in one case human parainfluenza virus (HPIV-3) or rabies virus (RABV) with the glycoprotein (GP) of one of the filoviridae have provided 100 % protection in macaques
  • Similarly, non-replicating recombinant human Adenoviruses and Venezuelan Equine Encephalitis Virus (VEEV) have also been used successfully in macaques.
  • Also 1 success with virus-like particles for EBOV
  • Naked DNA only in prime-boost with Adeno vector.

Some of these vaccine candidates include combinations of glycoproteins of different filoviridae (with some evidence of cross-protection) or GP + other structural proteins.

Note Crab-eating macaques = macaca fascicularis (Java aap)



Ep 318-3: Mehedy Abir Pathogenicity and virulence of Marburg virus Dec 2022

Transmission: origin from fruit bats is considered proven: Rousettus aegyptiacus species of bat most frequently acts as a reservoir of MARV, along with Hipposideros caffer and some unclassified Chiroptera as the minor sources



Transmission and spread of Marburg virus. Reservoirs of the Marburg virus, such as African fruit bats, can spread the virus among themselves by direct contamination, through sexual transmission, or due to biting. Direct contact with reservoir hosts or viral-contaminated fruit consumption may spread the virus to humans and non-human primates (NHPs). Transmission between humans and NHPs may occur through direct contact, and NHP-to-human transmission occurs due to bushmeat consumption and through direct contact. Direct contact and aerosols can facilitate both human-to-human and NHP-to-NHP transmission.

Clinical characteristics




  1. MARV entry, viral dissemination, and cellular tropism


The yellow color in the figure shows viral entry mechanisms, whereas the red color shows viral dissemination pathways. MARV enters the host and spreads throughout the lymphatic and vascular systems. The light brown color indicates the damaged cellular organelles. MARV causes necrosis in many organs, including the liver, spleen, kidneys, gastrointestinal tract, and endocardium.


  1. MARV hemorrhagic fever pathophysiology model in humans



Marburg virus primarily targets dendritic cells, monocytes, parenchyma cells at a liver, adrenocortical cells, and several lymphoid tissues. Infection of dendritic cells leads to poor stimulating condition of T lymphocyte that causes lymphocyte apoptotic condition. Due of this, body’s immunity is suppressed and cytokines/chemokines number is increased, which leads to shock as well as multiorgan damaging occurrence. Macrophage or monocyte infection leads to uncontrolled cytokines/chemokines activation, and they continue the damaging of T lymphocyte and endothelial cell. Endothelial cell infection causes increase of blood vessels permeability and DIC (disseminated intra-vascular coagulopathy), while both occurrences lead to hemorrhages. Systemic replication can also occur because of this infection in endothelial cells. Parenchymal cell infection occurrences in liver can decrease coagulation factors, and these occurrences can cause hemorrhages later on. Adrenocortical cells of adrenal gland infecting occurrence by MARV can lead to disorders in the metabolism and dysregulated blood pressure; and hemorrhage occurs at a later stage due to these infections. MARV infection the on lymphoid tissues of lymphatic system, especially lymph nodes and spleen infections lead to tissue necrosis and malfunctioning adaptive immunity. Shock and lymphoid organ damage can occur in the later stage.


Ep 318-4: Woolsey PLoS Pathogens Current state of EBOLA vaccines

The field of EBOV vaccines has been boosted by the large West-African outbreak in 2013-2016.  One replication competent VSV -based vaccine expressing EBOV glycoprotein (rVSVDG-ZEBOV-GP – ERVEBO Merck) has shown over 90 % protection in a phase 3 trial and has been approved for prophylactic use in humans by all major regulatory agencies.

A second vaccine from Janssen Zabdeno and Mvabea from J&J and Nordic Bavaria: heterologous prime with replication incompetent human Adeno26 expressing Zaire EBOV GP (Ad26-ZEBOV)  + boost with replication competent Modified Vaccina Ankara, expressing GP from Zaire EBOV, Sudan EBOV, Marburg and Tai Forest virus. (MVA-BN-Filo) has been approved by EMA, based on  animal and immunobridging data  (acquired during the West-African 2013-16 and RDC outbreak 2018)


Other vector-based vaccines by GSK, Cansino (China) and Gamaleya (Russia) have not yet been approved in US or EU.







The most advanced vaccines in the US and Europe include Ervebo (rVSV-EBOV), Zabdeno/Mvabea (Ad26-ZEBOV/MVA-BN-Filo),

and cAd3-EBOZ (with or without MVA-BN-Filo). These platforms use a viral vector to provoke an immune response, but, as illustrated, there are several distinctions among these 3 vaccines including vector virus, dose, efficacy, cell targets, and inclusion of a booster.


Ad26, human adenovirus serotype 26; cAd3, chimpanzee adenovirus serotype 3; EBOV, Ebola virus (Zaire ebolavirus); EBOZ, Ebolavirus-Zaire species; EMA, European Medicines Agency; FDA, US Federal Drug Administration; GP, glycoprotein; i.u., infectious unit; MARV, Marburg virus; NIAID, National Immunology Allergy and Infectious Disease; NP, nucleoprotein; PHAC, Public Health Agency of Canada; rVSV, recombinant vesicular stomatitis virus; SUDV, Sudan virus; TAFV, Taï Forest virus; ZEBOV, Zaire ebolavirus





Ad26, human adenovirus serotype 26; BIT, Beijing Institute of Technology; cAd3, chimpanzee adenovirus serotype 3; DRC, Democratic Republic of Congo; EBOV, Ebola virus; EBOZ, Ebolavirus-Zaire species; EMA, European Medicines Agency; EUAL, Emergency Use Authorization Listing; FDA, US Federal Drug Administration; GP, glycoprotein; HIV, human immunodeficiency virus; MARV, Marburg virus; NIAID, National Immunology Allergy and Infectious Disease; NP, nucleoprotein; rVSV, recombinant vesicular stomatitis virus; SAE, serious adverse event; SUDV, Sudan virus; TAFV, Taï Forest virus; Vx, Vaccination; WHO, World Health Organization; ZEBOV, Zaire ebolavirus.



Ep 318-5: John Suschak Hum Vacc Immunother 2019: Vaccines against Ebola virus and Marburg virus

The development of MARV vaccines are fully inspired on the EBOV vaccines, but have only been tested in NHP.



GP = glycoprotein; NP = nucleoprotein


Ep 318-6: Courtney Finch Vaccines 2022:  Vaccine Licensure in the Absence of Human Efficacy Data


FDA has adopted the “Animal Rule” under the following conditions:

  1. The product’s pathophysiological mechanism of toxicity or the disease and prevention by the product is well understood;
  2. The effect (e.g., protection induced by a vaccine) has been demonstrated in greater than one animal model thought to be predictive of human response (a single animal model may be sufficient in cases where the animal model is thoroughly characterized and considered highly predictive of the human response);
  3. The nonclinical endpoints are demonstrably associated with the desired benefit in humans (ex., a vaccine against a pathogen of high human case fatality that confers protection and yields survival in the animal model or a therapeutic that significantly improves disease signs and reduces recovery time);
  4. The data from the animal and human studies provide clear selection criteria for an effective human dose.


There may be post-marketing requirements: for instance, a post-marketing human efficacy trial is a likely requirement in the event that the opportunity arises for such a trial, e.g., in the event of an outbreak or declared emergency.


Alternatives to animal rule:

  • Accelerated Approval (US): effectiveness is demonstrated using a surrogate endpoint with a reasonable likelihood of predicting benefit in the clinic, e.g. an immune marker in the case of vaccines (e.g., seroprotective titer for chikungunya virus). In the absence of a marker reasonably likely to predict clinical benefit, the animal rule should be followed
  • Conditional Marketing Authorization (EMA):  if
    • the benefit of the product outweighs the risk,  
    • it is likely that a complete dataset (consistent with requirements for traditional approval) will be obtained postauthorization,
    • the product addresses an unmet need, and
    • the product offers immediate patient benefit that outweighs the risk associated with lacking a complete dataset
  • Exceptional Circumstances (EMA): if a Sponsor is unable to provide data on efficacy due to similar reasons described in the Animal Rule.  Example is the approval of the Janssen EBOLA vaccine Zabdeno/Mvabea vaccines in July 2020

Clearly, Conditional Marketing Authorization provides an opportunity to gather necessary human data for traditional

approval; however, if this is not possible, Exceptional Circumstances may be employed to achieve approval, but this approval will remain limited.


Ep 318-7: Kyle Shifflett Virology J 2019: Which animal model for Marburg?








Clearly, mice are easy to handle, but do not represent the human disease.  They can be used in the very early phase of vaccine development. Hamsters and guinea pigs have a more representative disease, but require the use of an adapted virus, which may decrease the relevance. Therefore, unfortunately for many reasons, showing effectiveness in NHP will be needed for the “animal rule” approval of new MARV (and other Filovirus) vaccines.




The viral vector vaccines against MARV are well advanced and they could take advantage of the “animal rule” in the US or similar exceptional regulations elsewhere to get conditional approval.  The very rare , but dangerous side effects of adenovirus vectors in the context of COVID vaccination (Thrombosis with Thrombocytopenia Syndrome (TTS) should not prevent their application in very lethal infections such as EBOV and MARV.  In  view of the success of mRNA vaccines in COVID, Moderna has announced the development of this type of vaccines for Filovirus infections as well (Ep 318-9). To be followed up….


Best wishes