7 December 2021 Episode 198 : Evidence for antibody dependent enhancement?

Tue, 12/07/2021 - 17:28

Episode 198 : Evidence for antibody dependent enhancement?

Dear colleagues,

As promised and based on constructive criticism by some of you, I investigated the question whether antibody-dependent enhancement (ADE) could have a role in COVID pathology: either during primary infection, reinfection or vaccination.   I performed a very thorough literature study.  There are many papers both in the scientific and lay press that speculate about this intriguing possibility.  As you will see, ADE exists as an in vitro lab phenomenon, but the evidence in animal models and the clinic is very weak until now.

Ep 198-1:  First, some thoughts by our colleague Rudi Westendorp, previously quoted for his enlightening comments on the different approach of COVID in Denmark versus The Netherlands.  Please read his cultural-historical perspective with the following concluding remark:

The corona issue is a typical unstructured problem - 'a wicked problem' - and difficult to solve because things constantly change and apparent contradictions occur, leaving no room for an unambiguous solution. The advance of corona is inextricably linked to immunity, vaccinations and hospitals, but also to economic growth, travel, traditions and customs. For example, the initial approach to the epidemic leads to new issues, in other words, in-depth knowledge and skills are indispensable, but simple solutions are not sufficient to parry the pandemic.

Constructive and Critical feedback

Besides overt support, some of you also raised questions about last week’s initiative to call for action in the primary school, including face masks and vaccination.  As could be expected from well-educated people, critical reactions were very reasonable.  Some were very elaborate and I want to share two of those for your consideration (each in the original language in A and translation into English in B):

Ep 198-2:  A colleague provided the transcript of an interview with Prof Jean-Luc Sabatier, Director of Research at the CNRS in Marseille, explaining the concept of “facilitating antibodies” (hence ADE) that could jeopardize our efforts to “vaccinate us out of COVID”;

Ep 198-3: A series of remarks by another colleague about natural immunity, antibody-dependent enhancement, usefulness of vaccines and face masks for children.  In addition a plea to use saline as a cheap alternative for prevention and treatment, with an impressive and well-documented  list of references in support of saline in Table 1 (Ep 198-3 C).

I have attached these contributions in order to encourage you to read them….    

News on children and vaccines

  1. Colleague Hans Snoeck sent the following message from New York:
    1. On an estimated 70 million children in the US between 5-11 yrs, 17% has received at least one dose of Pfizer and about 5 % two doses and New York City is leading.
    2. There have been no reports of serious adverse events published in the press
    3. In NYC only 5 schools have been closing this school year

For background see:       https://covid.cdc.gov/covid-data-tracker/#vaccination-demographics-trends





  1. Hans Kluge (WHO Europe) issued an advice to protect children from COVID


Faced with the rebound of the Covid-19 pandemic in Europe, the WHO called on Tuesday, December 7 to better protect children, currently the age group most affected by the virus.

According to the WHO, cases are currently increasing in all age categories, “with the highest rates currently observed among 5-14 year olds”. “It is not uncommon today to see incidences two to three times higher in young children than in the total population”, underlined Hans Kluge, director of the WHO in Europe.

To avoid further class closures and the return of distance education, the European branch of the organization advises to strengthen testing in schools and consider vaccination of school children.

“The use of masks and ventilation, along with regular testing, should be the norm in all primary schools and childhood immunizations should be discussed and considered nationally.”

Hans Kluge, Director of WHO in Europe

in press conference

As for compulsory vaccination, decided or envisaged by certain countries, it must remain a recourse of “absolute last resort, only when all possible options to increase the vaccination rate have been exhausted”, estimates the WHO. The organization has again expressed its concern over the new Omicron variant, but called upon to combat the current dominant variant, the Delta for “a victory tomorrow against Omicron”.

Antibody-dependent enhancement (ADE)

  1. Introductory remarks

Besides the two colleagues above, others also asked me about the possibility that ADE might be a pathogenic mechanism, especially in the context of waning responses after vaccination and the emergence of new variants. 

ADE has been described as a mechanism to increase viral load and pathology in Dengue and Feline Infectious Peritonitis Virus (FIPV).  In these cases, virus-binding, but non-neutralizing antibodies fail to block the virus, but promote infection of macrophages instead.  

Ep 198-4: Sridhar (NEJM 2018) shows that a tetravalent dengue vaccine fails to protect against severe dengue in previously non-infected (dengue-seronegative) children, while protecting those who had Dengue exposure before vaccination. 

It was known long before that primo-infection with one of the 4 Dengue serotypes and reinfection with a different serotype can also induce severe dengue manifestations.


Ep 198-5: Takano in JVMS 2018:

  • In FIPV, unlike dengue virus infection, ADE can be induced by re-infection with the identical serotype FIPV.
  • Even passive administration of antibodies, followed by oral infection can cause a peritonitis via ADE.


Ep 198-6: A similarly enhanced pathology was seen after Measles and Respiratory Syncytial virus

immunization of children with inactivated vaccines, that failed to elicit protective antibody and, in both cases, postvaccination exposure to wild type virus led to immune complex deposition in affected tissues, vigorous anamnestic CD4 T lymphocyte proliferative responses, and a Th2 bias of the immune responses.


Ep 198-8: Nice overview on antibody-dependent enhancement (ADE) in various viral diseases via two proposed mechanisms:

  • Enhanced infection via FcRII A mainly in macrophages, with the best example Dengue
  • Enhanced immune activation and inflammation via antigen-antibody complexes, with measles and RSC as best examples


Reminds that for SARS-CoV 1 and MERS it has been shown that neut monoclonal Ab can mediate uptake in monocyte-macrophages (MA) via FcRII1 .  However:

  • While MERS could replicate in MA,
  • This is not the case for SARS-CoV-1, which also fails to “activate” pro-inflammatory cytokine gene expression in MA
  • Uptake in MA can result in viral clearance in vitro and in vivo


These examples indicate that previous infection or vaccination, resulting in suboptimal neutralizing antibody titers can predispose to ADE phenomena with increased pathology upon (re)infection, but what is the evidence that this occurs in SARS-CoV-2 infection?

  1. In vitro experiments with cell lines or primary cells and polyclonal convalescent plasma


Ep 198-9 A: Clark bioRxiv Sept 2021: no evidence of ADE by convalescent serum IgA or IgG in cell lines expressing ACE-2, TMPRSS2 and either FcRα or FcRγIIA (= receptor for constant part of either IgA or IgG).


Ep 198-9 B: Maemura MBio Oct 2021,

  • Using a different cell line, expressing human FcRIIA or FcRIIIA and convalescent plasma: modest enhancement of infection,
  • Using human monocyte-macrophages (hMDM) and convalescent plasma
    • Increased infection
    • No increased cytokine production (hence no immune activation)


Ep 198-9 C: Garcia-Nicolas in Front Cell Infect Microbiol (April 2021): no evidence for ADE in vitro

  • hMDM express angiotensin-converting enzyme 2 (ACE-2), but no or very low levels of transmembrane protease serine 2 (TMPRSS2)
  • SARS-CoV-2 and SARS-CoV neither infect hMDM nor induce inflammatory cytokines in these cells, in contrast to Middle East respiratory syndrome (MERS) and the common cold human coronavirus 229E.
  • Convalescent COVID-19 serum neither induced enhancement of SARS-CoV-2 infection nor innate immune response in hMDM.


Therefore no indication of a link with the COVID-associated inflammation


  1. In vitro and in vivo experiments with monoclonal antibodies from convalescent plasma


Ep 198-10 A: Yunjiao Zhou Cell Rep Feb 2021 analyzes 48 human monoclonal antibodies (hu mAb),  derived from convalescent B cells. They find receptor-binding domain (RBD) antibodies targeting  four groups of non-overlapping epitopes:

  • Group IV Ab induce  ADE of entry in Raji cells via FcR;
  • Group II/III antibodies neutralize SARS-CoV-2 without mediating ADE of entry


Ep 198-10 B: Dapeng Li Cell Aug 2021 on in vitro and in vivo neutralization and enhancement by hu mAbs:

  • Selection of large numbers of hu mAbs from patients, reactive with either the RBD or N-terminal domain (NTD) of SARS-CoV-2 Spike.
  • In vitro many mAbs showed neutralization, some showed enhancement that was either FcR dependent or not.
  • In vivo (macaques) 45 of 46 spike enhancing Ab-infused monkeys did not show enhanced virus replication, but  3 of 46 Ab-treated monkeys exhibited enhancement of lung pathology, and 1 of 46 Ab-treated monkeys had alveolar and perivascular edema with elevated broncho-alveolar inflammatory cytokines


It seems that in vivo enhancement at the mAb level is a (very) rare phenomenon and could (easily?) be offset by other antibodies in the polyclonal population with strong neutralizing activity. 


The reason why in vitro enhancing Ab are rather neutralizing in vivo can be related to other FcR-dependent functions, like antibody-dependent cytotoxicity by natural killer cells or macrophages.  


Ep 198-10 C: Nouara Yahi J Infect Aug 2021 (p. 619-20) investigates the molecular mechanism why one of the NTD-specific mAbs can enhance infection and suggests that the delta variant may be more susceptible to this effect.  They conclude that this possibility should be further investigated, because they fear that vaccines, based on Wuhan could elicit delta-enhancing Ab.

Nice paper, but certainly not conclusive as to the risk on ADE in patients or vaccinees. 


  1. Effect of vaccination on ADE ?


Ep 198-11: Dandan Li in Emerg Microb Infect Dec 2021: No ADE elicited by waning Ab after inactivated vaccine.

Concerns of ADE, especially when the neutralizing antibody levels induced by vaccination or initial infection quickly wane to sub-neutralizing levels.

Nevertheless, passive transfer of diluted polyclonal antibodies in macaques, elicited by inactivated vaccine,  could still provide some level of protection against infection upon challenge, and no antibody-enhanced infection was observed.


Ep 198-12: A comment by Amal Hasan in Hum Vacc Immunother Sept 2021 “Early insight into antibody-dependent enhancement after SARS-CoV-2 mRNA vaccination”. Despite this title, there are no hard data to substantiate the claim of the title.  Fig 1 is on early data from England, suggesting a higher death rate by Delta in fully vaccinated subjects, but that effect has been absent in all later reports, while, on the contrary, a very high protection against hospitalization and death has consistently been shown. 


Ep 198-13: Jiaxin Zheng in CMI (Oct 2021) gives a nice overview on humoral responses in disease and vaccination.  The very last paragraph warns:   “the presence of sub-neutralizing or cross-reactive non-NAbs has the theoretical potential to enhance infection and trigger harmful immunopathology”, but no data are provided.


  1. Role of ADE in Multisystem Inflammatory Syndrome in Children  (MIS-C) ?


Ep 198-14: Salvatore Panaro Front Med Oct 2021 gives an exhaustive overview of the various proposed pathogenic mechanisms for this syndrome (none of which has definitely been proven):  

  1. Defective dendritic cell function: insufficient type 1 IFN by plasmocytic DC and insufficient CD8 T cell activation by conventional DC. 
  2. Superantigen on Spike that activates auto-reactive T cells in children with particular T cell receptors (genetically defined).
  3. Lack of potentially protective cross-reactive Ab against common cold CoV (HKU-1).
  4. Auto-antibodies e.g. anti-endoglin resulting in endothelial damage.
  5. ADE, since MIS-C symptoms are somewhat reminiscent of severe Dengue.


Ep 198-15: The theme of ADE has been worked out further by Darrel Ricke Front Immunol Feb 2021. He presents a very short molecular analysis of variation in the Spike protein and then goes on to speculate about pathogenic mechanisms of COVID-19 and MIS-C in particular. Two different ADE pathways are suggested:

  1. The classical viral-Ig complexes that could infect macrophages and other FcR (+) cells
  2. Degranulation of mast cells, with massive release of histamine, consistent with MIS-C in infants with maternally transferred antibodies.

Unfortunately, no hard data are presented to substantiate either of these hypotheses.


Ep 198-16: Leal Yonker J Paediatrics Dec 2021 present a series of 192 children with suspected SARS-CoV-2: of these 49 with acute COVID and 18 met the MIS-C criteria. Interesting observations:

  • Elevated levels of IgG and IgM Ab to Spike RBD in acute SARS-CoV-2 and MIS-C
  • Higher Ab levels in severe vs mild MIS-C
  • Polyclonal Ab in severe MIS-C: also against Common cold CoV, RSV, Flu
  • Positive correlation between  SARS-CoV2 serology and inflammatory markers ferritin and NT-ProBNP (N-terminal pro hormone B-type natriuretic peptide).

Interpretation of authors:  

  • The broad, nonspecific antibody response points to T- and B-cell over-reactivity, or to auto-antibodies that may be driving an inflammatory process causing MIS-C.
  • Elevated ferritin levels in MIS-C, which positively correlate with SARS-CoV-2 serology, also suggest an interplay with macrophage activation.
  • Further, SARS-CoV-2 IgG are positively correlated with NT-proBNP, a marker of heart failure, which could indicate mechanism of disease.

Remark: It is possible but not proven that ADE has a role in this proposed mechanism


  1. Role of ADE in reinfection?


Ep 198-17: Lo Muzio In In J Env Res Pub Health Oct 2021 provides a very thorough systematic review, based on a strict CDC definition of reinfection (Table 1),  resulting in 260 patients from 117 studies.  Some highlights:

  • Frequency of reinfection: really exceptional  0.1-2 %
  • In 2/3 of cases less symptoms, but in 35.5 % more severe, with appr 5 % deaths.
  • Different viral clades in 20 %
  • Men 60 %; most in age 20-60 yrs; 40 % healthy subjects, 35 % co-morbidities, 25 % HCW



  • In 2/3 of the cases who are immunocompetent: waning immunity and/or high exposure (HCW) is the most likely explanation.
  • In 1/3 with more severe symptoms  “enhancing” is a possibility.   


Persistent infections: only described in immune-deficient patients.


Ep 198-18: Milne in Lancet Resp Med Oct 2021: very nice and didactic overview


  • Humoral response: duration and breadth varies markedly by age and disease severity, with  detectable neutralizing antibodies, responses present for up to 1 year after infection; but memory B may persist longer
  • T cell responses more poly-epitopic and also persisting for 5-8 months: may mitigate disease severity upon rechallenge, especially  in younger people.
  • Reinfection occurs  approximately 5–12 months, with individuals who were initially seropositive for IgG antibodies having therefore a lower risk of reinfection
  • Given the considerable viral epitopic mutation, it is likely that SARS-CoV-2 vaccines will need to be updated on a seasonal or yearly basis to maintain population-level protective immunity, as is the case with other endemic respiratory viruses;
  • Other interventions might also be required to prevent the occurrence of further significant outbreaks and reduce the incidence of disease


Ep 198-19: Fakhroo in Vaccines MDPI Oct 2021 reiterates on a didactic overview of possible causes for reinfection:

  • Viral:  escape mutant or high dose challenge;  (virus hiding in the body is rather in persistent or recurrent infection i.e. within days or weeks, not months)
  • Immunity:  
    • Immune deficiency; low  initial response (asympt or mild infection); waning response
    • ADE: low affinity Ab and new variant → severe disease


Clearly, ADE is a possible (not proven) mechanism involved in 1/3 of reinfection cases, which constitute 1 % of all patients.  Hence rather rare….  


Ep 198-20 A and B: Update on omicron and reinfection risk


As compared to the reinfection risk during the first (Wuhan) wave the chances of reinfection have increased for omicron (Hazard Ratio 2.39), while for beta and delta this risk was actually decreased (HR 0.75 and 0.71), compared with Wuhan.   



  • Population-level evidence suggests that the Omicron variant is associated with substantial ability to evade immunity from prior infection.
  • In contrast, there is no population-wide epidemiological evidence of immune escape associated with the Beta or Delta variants.


Urgent questions

  • whether Omicron is also able to evade vaccine-induced immunity
  • implications of reduced immunity on protection against severe disease and death.


Some general conclusions


  1. The ADE phenomenon can be observed with monoclonal antibodies (derived from convalescent patients B cells) on infection in cell lines, it is less reproducible in primary human macrophages and, until to date, there is very little evidence that mAb can reproduce ADE in animal models.


  1. It is possible, but not proven, that ADE has a role in primary severe COVID disease: through low affinity Ab (potentially elicited by cross-reactive common cold CoV?) that enhance immune activation and inflammation (while hyper-infection of macrophages is less likely).

This could be the case for elderly, but also for children or adults with MIS.


  1. ADE is one of potential mechanisms to explain severity of “secondary’ infection: i.e. in the minority of patients, who present with severe symptoms during reinfection. 

Interestingly, omicron is associated with increased frequency of reinfection.  


  1. There is no evidence that ADE has a role in vaccine breakthrough infections, as these are generally mild.     


For all these reasons, my feeling is that hesitancy to vaccinate either adults or children, based on fear for ADE, is not based on sound scientific evidence today.


Of course, it cannot be excluded that a new viral variant (such as omicron) would behave differently and today it is unknown. Hopefully we will learn more soon from the South African epidemiological data and from in vitro neutralization.   


Best wishes,