Introduction
Vaccines are the best cost-benefit tools to control and eradicate infectious diseases.
The live smallpox vaccination, called variolation, was the injection of the homologous
virus and this promoted self-healing local lesions that guaranteed strong and long-lasting
protection. However, since 3% of these variolations caused cases of smallpox in the
vaccinated individuals, it was considered unsafe and was discontinued (1–5).
In 1796, Edward Jenner, who had been variolated, discovered the vaccination principle
when he used the cowpox virus live-vaccine (vaccinia virus) to induce cross-immunity
and prevent human smallpox in a child. His strong merit was to initiate the campaign
that turned vaccination against smallpox obligatory and universal, and to discover
that cross-protection promoted by a heterologous, although related, organism was sufficient
to guarantee efficacy and reduce the safety issues of the homologous live-vaccine
(6).
In 1967, due to the World Health campaigns, smallpox was considered the first and
only human viral infection ever eradicated (4, 5). Ironically, Jenner never knew that
smallpox was induced by a virus (6, 7) which suggests that, what is needed for the
eradication of a disease is the systematic worldwide use of a potent and efficient
vaccine.
After the Sabin anti-polio vaccine, which was launched in the 1960's, many other vaccines
have been developed based on whole attenuated viruses. However, poliomyelitis was
also induced by the Sabin vaccine poliovirus 2 in healthy subjects (8–10). This means
that, nowadays, live-vaccination with whole wild or attenuated virus is no longer
ethically possible, mainly because of the large population of immunocompromised subjects,
in which a live-vaccine could cause the disease. Due to these safety issues, whole
virus and bacterial dead or inactivated vaccines have progressively substituted live-vaccines.
We could guess that this is precisely what Louis Pasteur, the father of Microbiology,
would have done to control and eradicate COVID-19. Although he worked initially with
the attenuation of viruses and bacteria, after his successful work with Rabies, fowl
cholera and Anthrax (11), it became clear three steps were needed to develop a protective
vaccine against infection. First, the organism should be isolated, then inactivated,
and finally injected (5). In 1885, Pasteur's rabies vaccine employed an air dried
fixed virus. Semple improved the fixation by adding phenol (12). Currently β-propiolactone
is considered to be better than phenol or formaldehyde. However, it is carcinogenic
(13). Therefore, other methods like ultraviolet or gamma-irradiation, high pressure
(14), visible ultrashort pulsed laser, and low-energy electron irradiation have been
suggested (15).
Advantages of the Whole Inactivated Virus Vaccine
The most important advantage of whole inactivated vaccines is that, unlike the live
or attenuated vaccines, they do not cause the disease (Table 1). In fact, inactivated
vaccines preserve the intact structure of the antigens and their B-cell epitopes that
enable them to interact with the antibody paratopes, and promote the synthesis of
neutralizing antibodies. They can not only stimulate the humoral, but the cellular
immune responses as well, in a manner similar to live viruses, since they preserve
the virus structures during inactivation (15). Cross-presentation of conserved epitopes
to the cytotoxic T cells (CTL), through the major HLA class 1 histocompatibility system,
in addition to the viral pathogen associated patterns (PAMPS), which use innate immune
receptors such as Toll like receptor 7, can induce T cell mediated responses. In fact,
in the late endosome of the infected APC three different events can occur: (1) viral
degradation following the exogenous pattern and presentation to CD4 T cells via the
MHC class II molecules, (2) cross presentation pathway to CD8 T cells via the MHC
class I molecules, and (3) viral membrane fusion following the endogenous pathway.
The above mentioned pathways along with the recognition of viral PAMPS, using PRRs
such as TLR7, as well as production of cytokines such as IFN-1 can promote potent
cellular mediated immune responses (15). However, in the case of Influenza vaccines
for instance, the inactivated formulations may not always induce T cell responses
as potent as the live-vaccines. In fact, the inactivated vaccine may even prevent
or suppress the induction of cross-reactive CD8+ T-cells (16).
Table 1
Evolution of vaccine compositions.
Types of vaccines
Live or attenuated
Inactivated
Exotoxins
Recombinant and DNA vaccines
Neutralizing antibodies
Yes
Yes
Yes
No
Reversion to virulence
Yes
No
No
No
Protection
High
Low
Low
No
CMI (TH1)
Strong
Yes
Yes
No
CTL (CD8)
Strong
Yes
Yes
No
Disease in immunosuppressed
Yes
No
No
No
Contain DNA, LPS, or other PAMPS
Yes
Yes
Yes
No
Requires adjuvants
No
No
No
Yes
PAMPS are constitutive components of the virus and bacteria (viral or bacterial nucleic
acids, polysaccharides, Lipopolysaccharides, Lipid A, monophosphoryl lipid A, bacterial
peptidoglycans, etc.). They are compounds universally recognized by the innate immune
system of healthy subjects, who build a natural protective response against them.
In contrast, modern purified, fractionated, recombinant, or synthetic vaccines gained
in safety but lost potency because they lack the PAMPS (Table 1). While, vaccines
using inactivated organisms with PAMPS have shown great success against polio, whooping
cough, and tetanus (17, 18).
If we use fixation of the structures of some of the isolates that cause the disease,
then inactivate them and preserve their whole structures, the possible deleterious
effect of the high mutagenicity detected in a few of the proteins of the virus (19)
would be overcome by the strong immune response generated against the whole virus
structure. Therefore, the mutagenicity should not be critical for the generation of
protection, and would not damage the efficacy of the whole vaccine. Furthermore, it
might be that the whole virus inactivated vaccine could even induce some cross protection
against other Coronovirus agents of SARS, which hold conservative structures (20).
Furthermore, whole inactivated vaccines are considered good candidates for designing
universal vaccines capable of giving protection against multiple strains of Influenza
virus (15).
It is true that an impressive amount of data about the DNA sequencing of the virus
has been gathered in a relatively short period of time and with that, the knowledge
of its biological properties increased enormously (23,927 sequences in PUBMED) (19,
21–26). However, for an urgent strategy, we could also take advantage of the lessons
taught by the history of vaccinology in order to prevent the disease and save lives.
Furthermore, if we want to enhance the efficacy of the vaccine we should combine the
inactivated virus with a good adjuvant. The adjuvant might contain saponins of Quillaja
saponaria Molina, which induce antibodies of desired subtypes, promote both the cytotoxic
antiviral CD8+ and CD4+ Th1 cell responses against the infection and that been used
with success in vaccines against leishmaniasis (7, 27, 28), cancer (29), Malaria (30),
Herpes zoster (31), and HIV (32).
In spite of the valuable guidelines from the work of Jenner and Pasteur, who with
much fewer resources, developed vaccines that controlled and eradicated smallpox that
showed a 40% mortality rate (33) and rabies with 100% mortality rate (34); and even
with all the knowledge acquired since then, there is no urgent combined international
effort to produce one unique vaccine. Instead, 6 months after the description of the
first COVID-19 cases in China, 147 vaccines are reported to be in development all
over the world and, only two of them contain the inactivated virus (35, 36). All the
other formulations include live attenuated, non-replicating or replicating viral vector,
recombinant protein, peptide base, virus-like particle, and virus DNA and RNA. Most
of these vaccines are focused on only one antigen of the Coronavirus, therefore, these
formulations will certainly be less potent than a vaccine made up of multiple antigens
contained in the whole pathogen.
Furthermore, many of these formulations do not use the technology involved in any
previously licensed vaccines (37). CEPI (Coalition for Epidemic Preparedness Innovation)
estimated the development of Phase I clinical trials of 8 vaccines, Phase 2 and 3
trials for up to 6 vaccines and progression to regulatory approval and production
of up to 3 candidates (38).
In fact, by May 11th, 2020 seven vaccines had already entered Phase I clinical trials:
(1) encapsulated mRNA encoding protein S (Moderna and NIAID, USA); (2) Adenovirus
expressing protein S (Cansino Biologics, China); (3) DCs modified with lentivirus
expressing several proteins and CTLs (Shenzen Geno-Immune Medical, China); (4) an
APC modified with lentivirus expressing several viral proteins (35); (5) Inno 4800,
SARS CoV2 DNA Injection (Innovio, USA); (6) ChAdOx1 vaccine from the Jenner Institute,
Oxford University, (UK) which is a genetically modified Adenovirus expressing Coronavirus
proteins (39), and is also being tested in a Phase II trial; and finally (7) the whole
inactivated coronavirus with Alum by Sinovac, China (40). Furthermore, on July 2nd,
2020 the WHO communicates that there are 18 COVID-19 candidate vaccines in clinical
evaluation and more 129 under pre-clinical assays (36).
Current Vaccines With Published Results of Preclinical Evaluation, Under Phase III
Clinical Trials and Large-Scale Production
Only one of the vaccines under clinical trials is currently supported by a peer reviewed
scientific publication in Science that was published on May 7th, 2020: the inactivated
whole virus vaccine of Sinovac (40). The results of its pre-clinical assays in the
mouse, rat and non-human primate model were published before, without peer review
on April 13th in the bioRxiv. Later on, on May 13th, the results of the Chadox1 adenovirus
vaccine of the Jenner Institute of Oxford University were published with no peer review
in the bioRxiv (39). Until June 29th, there has been no peer reviewed publication
of this vaccine.
Regarding the formulations, the inactivated whole virus Sinovac vaccine is composed
of one isolate of Sars-CoV2 (CN2) obtained from a patient of China and Alum adjuvant
(40), while the Chadox1 nCoV19 vaccine of Oxford is composed of a Chimpanzee recombinant
adenovirus, which expresses the S protein of SARS-Cov2 (39).
Sinovac Biotech (China) in collaboration with several Universities, Public Health
institutions and the Medical Academy of the Army of China have been able to produce
a whole virus inactivated vaccine adjuvanted by alum that was stable and showed 99.8
to 100% sequence identity to 10 other isolates also obtained from broncheoalveolar
fluid (BALF) of hospitalized patients (five in intensive care), from China, Italy,
United Kingdom, Switzerland and Spain (40). The virus was propagated in cultures of
Vero cells in vitro and inactivated with β-propiolactone (40). The use of Alum adjuvant
is approved for human vaccines because it induces strong antibody responses, mainly
of the IgG1 and IgE types that show efficacy against virus or bacterial diseases,
which need neutralizing antibodies to be controlled. However, alum is a poor promoter
of the cellular immune responses against pathogens (41).
In contrast, the Chadox1 nCoV-19 vaccine developed by Oxford University and AstraZeneca
is composed of a recombinant non-replicant chimpanzee adenovirus, which expresses
the S protein of Sars-CoV-2 (39). Different from the technology used for inactivated
vaccines since the 1800's, this adenovirus platform was developed in 2012 (42). The
authors aimed to include an adenovirus in the vaccine that would not infect humans,
in order to avoid its potential rejection by human antibodies. The chosen Chimpanzee
adenovirus was phylogenetically related to the human adenovirus. The inventors deleted
the region E1 of the chimpanzee adenovirus genome in order to render the virus defective
and non-replicant, while the E3 region was excluded to increase the insert capacity.
In addition, a bacterial artificial chromosome (BAC) containing a codon-optimized
full-length spike protein of SARS-CoV-2 with a human tPA leader sequence (39) was
added between the deleted E1 region and E4 to facilitate the genetic modifications.
This approach has been reported to improve genetic stability (42). However, additional
modifications were needed to guarantee that the E4 region of the virus would express
a human, instead of a simian protein, that would enable the virus recognition and
propagation inside human cells in in vitro culture, for large-scale virus production
(42).
Regarding the number of samples, the SINOVAC inactivated vaccine was tested in groups
of 10 mice and 10 rats and in 4 cohorts of 10 monkeys (Macaca mulatta) (40), while
the Chadox1 nCOV-19 vaccine was only tested in groups of 5–8 mice and in 6 Rhesus
monkeys using only 3 monkeys as controls (Table 2) (39).
Table 2
Comparison of efficacies of vaccine candidates with published results.
Results and end-points of efficacy
Whole virus inactivated vaccine (SINOVA)
Recombinant chimpanzee adenovirus expressing the S antigen (Chadox1 nCOV19)
Sample size in mice
4 Groups of 10
5–8
Sample size in rats
4 Groups of 10
-
Sample size in monkeys
Groups of 4 and 4 groups of 10
6 Vaccinated x 3 controls
IgG anti-S antibodies in mice
High
High IgG2a, IgG2b, IgG1
IgG anti-RBD antibodies in mice
High
Nd
IgG anti-N antibodies in mice
Intermediate
Nd
Neutralizing antibodies in mice
High
3/5
Neutralizing antibodies in rats
High
Nd
Mice S-specific CD4+ and CD8+ Th1 T cells expressing
Nd
IFN-γ, TNF-α
Mice S-specific CD4+ and CD8+ Th2 T cells expressing
Nd
IL-4
Mice S-specific Th1 cytokines in supernatants
Nd
IFN-γ, TNF-α, IL-2
Mice S-specific Th2 cytokines in supernatants
Nd
IL-6
IgG anti-S antibodies in monkeys
High
High
IgG anti-RBD antibodies in monkeys
High
Nd
IgG anti-N antibodies in monkeys
Low
Nd
Virus neutralizing antibodies in monkeys
High
High
Th1 cytokines in sera of vaccinated monkeys
No variation of IFN-γ, TNF-α, IL-2
IFN-γ 6/6 and TNF-α in 1/6
Th2 cytokines in sera of vaccinated monkeys
No variation of IL-4, IL-5, IL-6
IL-6 1/6 and IL-10 in 1/6
CD3 lymphocytes
No variation
Nd
CD4 lymphocytes
No variation
Nd
CD8 lymphocytes
No variation
Nd
Cross protection to other SARS-CoV2 isolates
High
Nd
Viral load in nasopharynx
No
Yes
viral load in BALF
No
1/6
viral load in lungs
No
Partial
viral load in anal swabs
No
Nd
Safety in monkeys
Yes
Yes
Prevention against infection in monkeys
Yes
No
Prevention against disease in monkeys
Yes
No
Prevention against severe disease in monkeys
Yes
Yes
Prevention against mortality in monkeys
Nd
Nd
Nd, not determined.
Regarding the antibody response in mice and rats, the Sinovac vaccine promoted high
IgG antibody titers against protein S, against its Receptor Binding domain, and to
a lower extent against protein N (Table 2) and also high titers of virus neutralizing
antibodies. The cytokine expression induced by the inactivated vaccine in mice was
not analyzed (40). In contrast, the Chadox1 vaccine induced anti-S1 and S2 protein
IgG antibody titers (IgG2a, IgG2b, and IgG1) and neutralizing antibodies in only three
of the five BALB/c mice, but showed IFN-γ, TNF-α, and IL-4 expressed by CD4+ and CD8+
T cells and IFN-γ, TNF-α, IL-2, and remarkably IL-6 secreted to the supernatants (39)
(Table 2).
Furthermore, in vaccinated monkeys, seven days after infection, the Sinovac inactivated
vaccine at 6 μg/dose induced high titers of IgG antibodies directed against the S,
RBD and lower levels of anti-N protein antibodies, high titers of virus neutralizing
antibodies with no detected antibody-dependent enhancement of disease (ADE) (40).
In contrast, anti-S protein IgG and neutralizing antibodies were detected in the 6
Rhesus monkeys vaccinated with Chadox1 (Table 2) (39). Moreover, regarding the concern
of the increased pro-inflammatory events and cytokine storm related to the severity
of COVID-19, the Sinovac vaccine was safe and did not promote any alteration in the
frequencies of CD3+, CD4+, or CD8 T cells nor of secretion of IFN-γ, TNF-α, IL-2,
IL-4, IL-5, or IL-6 (Table 2) (40). In contrast, increased levels of IFN-γ, TNF-α,
IL-6, and IL-10 were observed in monkeys vaccinated with the Chadox1 vaccine (39)
(Table 2) in which the frequencies of cytokine secreting T lymphocytes was not studied
(Table 2).
Regarding cross-protection to other SARS CoV2 isolates, the Sinovac inactivated vaccine
protected mice and rats against the challenge with 11 different virus isolates (Table
2) suggesting its potential use all over the World (40). There is no available data
concerning cross-protection for the Chadox 1 vaccine (39).
Besides, for a fair comparison of vaccine efficacies, the two SARS-CoV2 vaccines should
be assayed in the same field trial, and the efficacy end-points should be determined
prior to the assay. Due to the urgency in saving lives, this might be not be feasible
during the pandemic. However, for comparative purposes, early infection, disease,
severe disease, and death due to COVID-19 or other causes should be recorded as vaccine
efficacy end-points. For instance, reduction of the virus load in the nasal and pharynx
mucosa indicates not only protection against early infection, but also the blockade
of the transmission of infection by respiratory droplets. This means that this end
point is particularly important when seeking a vaccine to interrupt the epidemic.
In addition, clinical symptoms indicate disease, while pulmonary distress, cytokine
storm, need of intensive care, intubation indicate severe disease. The number of deaths
due to COVID-19 or other causes should also be recorded and compared in order to evaluate
the reduction of mortality.
Notable, the Sinovac inactivated vaccine reduced to zero the viral load in throat
swabs (pharynx and crissum), anal swabs and all regions of both lungs of vaccinated
and challenged monkeys (40) indicating that the inactivated vaccine prevents not only
the early infection but also blocks the transmission of the disease by droplets curtailing
the epidemics. In contrast, no viral sgRNA indicative of viral replication, could
be detected in BAL fluids, and in the lungs of two of six monkeys vaccinated with
Chadox 1 and challenged (39) (Table 2). In fact, the lung viral load decreased by
~60% in monkeys vaccinated with Chadox1. However, viral gRNA was detected in nose
swabs of all vaccinated and challenged animals (Table 2) (39), indicating that the
Chadox1 vaccine would not prevent the SARS CoV2 human infection nor block its transmission
and interrupt the epidemics. Vaccinated and infected subjects will continue to be
infectious and spread SARS CoV-2. However, the vaccine will probably, in most cases,
reduce the pulmonary symptoms, and make the disease less severe. Accordingly, the
inventors of the Chadox1 vaccine seem to be aware of the limitations of its efficacy
when they describe it as a vaccine that prevents pneumonia in monkeys (39).
Unfortunately, neither the investigations of the Sinovac nor the Chadox1 vaccine have
disclosed if any of their formulations prevent or reduces mortality.
Furthermore, in a report that analyzes the first results of the vaccine trials, published
in Nature, Peter Hotez considered that the Oxford vaccine induced very modest titers
of neutralizing antibodies and that considerable higher titers would be needed to
afford protection (43). At the same time, Hotez also says that the Sinovac vaccine
elicited a more promising antibody response in macaques monkeys (43).
In spite of that, WHO disclosed that this vaccine is in fact being tested in UK in
Phase I, II and III trials (36) and will be tested in a Phase III trial in Brazil
on 2,000 volunteers. Consequently, contracts for large-scale fabrication have already
been signed with the Public Laboratory of the Brazilian Ministry of Health Bio-Manguinhos.
In Brazil, 30 million doses are intended to be produced by Bio-Manguinhos and another
100 million after the proven efficacy of the vaccine. At this point it is important
to know which end-points of vaccine efficacy will be taken into consideration for
such an important decision.
On the other hand, the Sinovac whole virus inactivated vaccine was also reported to
have been successful in Phase I and II trials in 18–59 year olds (n = 422) and in
healthy elderly adults >60 years old (n = 744) in China (36) although these results
have not yet been published in detail. More than 90 % of the volunteers showed neutralizing
antibodies (44). A recent contract has been signed between Sinovac and the public
Laboratory Instituto Butantan of São Paulo, Brazil, in order to produce doses of the
vaccine to immunize 8,870 healthcare professionals for a double-blind randomized Phase
III trial in Brazil, where the incidence of cases and deaths due to COVID-19 is still
high (45). The results of the efficacy are expected in October 2020. In return, the
Instituto Butantan will gain the transfer of the technology and the license to manufacture
60,000,000 doses for Brazil. Testing anti-COVID-19 vaccines in Brazil became interesting
because of the high morbidity and mortality and active expansion of the epidemics.
An important warning is given by Ewen Callaway in his article published in Nature
(43), in which he asks for caution about the potential success of vaccines that arise
from small animal or human studies. This might be the case of the Moderna-NIAID vaccine
composed of lipid nano-particle encapsulated synthetic mRNA, which encodes the spike
S protein and already underwent Phase I and II clinical trials in USA (36). Moderna
company announced that Phase III trials are predicted to start in July 2020 and that
studies in monkeys are underway in parallel. None of these mRNA based vaccine has
ever been licensed before (43).
We conclude that the first results of anti-COVID-19 vaccine candidates confirm that
the whole virus inactivated vaccine, which preserves the immunogenicity of all the
antigens of the virus and contains PAMPS (40) is more potent than the recombinant
vaccines that have only the important S spicula protein, either expressed by an engineered
adenovirus (39) or by LNP encapsulated mRNA (43). Furthermore, the inactivated vaccine
also contains the Alum adjuvant. There are other examples support the superiority
of whole inactivated vaccines above those expressing recombinants single antigens.
For instance, 7.5 μg/dose of the trivalent inactivated Influenza vaccine is as safe
as, but more immunogenic than the 22.5 μg/dose of the recombinant baculovirus-expressed
hemagglutinin FluBok vaccine, in young children (46).
This is a fast moving scenario and several Phase 1 clinical trials of COVID vaccines
have been published, either with or without peer reviews. Two recombinant adenovirus
vaccines expressing the S spike protein of SARS-CoV-2, the Chadox1 and the Cansino
vaccines (47, 48), and two other vaccines composed of mRNA codifying for the S-protein
(mRNA1273, Moderna vaccine) (49) or its RDB domain (mRNA BNT162b1 Pfizer-Biontech
vaccine) (50, 51) have been assayed for safety and immunogenicity in Phase I-II clinical
trials in humans. There were no serious adverse events related to any of the four
vaccines (47–51). Local and systemic reactions commonly including pain, feeling feverish,
chills, muscle ache, headache, and malaise were recorded for all formulations (47–51)
and were reduced, in the case of the Chadox1 vaccine, with use of prophylactic paracetamol
(47). Only the Cansino vaccine was given as a single dose (48) while Chadox1, Moderna,
and Pfizer Biontech vaccines were assayed in two-dose protocols (47, 49–51). Anti-S
protein IgG responses rose by day 14 (47) and peaked or increased by day 21–28, after
the first (48) or second immunization dose, respectively (47, 49–51). In addition,
spike-specific T-cell responses detected by an ex-vivo interferon-γ enzyme-linked
immunospot assay, peaked on day 14 for the Chadox1 (47) and on day 28 for the Cansino
adenovirus vaccine (48). Moreover, the Moderna mRNA-1,273 vaccine induced a Th1 response
against the S-protein peptide pools (TNF-α >Il-2 >IFN-γ), with a minimal Th2 cytokine
expression (IL-4 and IL-13) and with CD8 T-cell responses, only detected at low levels,
after the second vaccination (49). In agreement, most participants vaccinated with
the Pfizer-Biontech mRNA-RBD vaccine (BNT162b1) also had Th1 skewed T cell immune
responses with RBD-specific CD8+ and CD4+ T cell expansion and IFN-γ produced by a
high fraction of RBD-specific CD8+ and CD4+ T cells (51). Furthermore, the levels
of neutralizing antibodies raised by each one of the vaccines could be considered
as correlates of their potential efficacy. While the mRNA-1273 of Moderna disclosed
50% EC values ranging between 256 and 512 (49), the maximal titer for the mRNA RBD
vaccine of Pfizer-Biontech was 308 (51) and for the Chadox1 vaccine, from 256 to 512
(47). The Cansino vaccine expresses its results as GMT (4–55, 61) impeding an accurate
comparison (48). Unfortunately, the results of Phase I-II clinical trial of the whole
virus inactivated vaccine of Sinovac have not yet been published in detail, therefore
although the vaccine was tested in the largest number of individuals (n = 1,166) (36),
a fair comparison of the safety and immunogenicity results is not yet possible. Ultimately,
only the results of the Phase III trials will disclose the potential impact of the
vaccines on reduction of deaths, clinical cases, and virus particles or viral RNA
in nasopharynx and will allow their efficacy and capability to interrupt the epidemic
to be evaluated.
Discussion
In the imminence of a pandemic involving high mortality and economic distress, several
factors could speed up the development of vaccines. One of them would be the use of
an already standardized methodology. It is worth noting that most of the molecularly
defined vaccines now in development would meet severe restrictions for large-scale
production and this could led to an enormous delay to deliver vaccines for mass vaccination
of the public. In contrast to this, nowadays large industries and public laboratories
are authorized to produce inactivated vaccines against Influenza. It is also reasonable
to hypothesize that generation of protection and immunological memory against a group
of antigens will be more efficient than that generated against a single antigen, no
matter, how important it is.
Two concerns could be considered as the downside of the inactivated vaccines for SARS
diseases. The first would be the fear of an incomplete inactivation of the virus that
could cause outbreaks among the vaccine production workers or in vaccinated populations
(52). This concern is common to all vaccines produced with native antigens, which
demand the production of large mass of pathogens. However, to guarantee safety, each
batch of vaccine is submitted to validation of inactivation controls that include
sequential passages assays of residual virus infectivity in embryonated eggs or tissue
culture, and detection of live virus by TCID50 assays (14). The whole virus SARS-CoV-2
inactivated vaccine of Sinovac includes validation of inactivation controls (40).
The second concern would be the promotion of an Antibody Disease Enhancement syndrome
(ADE) by the vaccine. This is usually related to non-neutralizing antibodies, which
determine an increased lung pathology and it was observed before in vaccines against
RSV and Measles in the 1960's (53). Since SARS-CoV-1, MERS-CoV, and SARS-CoV-2 are
phylogenetically related viruses that have caused epidemics over the last 16 years
and ADE pathology was present for some SARSCoV-1 and MERS vaccine candidates in animal
models, there is also a concern about the induction of ADE syndrome in humans vaccinated
with SARS-CoV-2 vaccine candidates (53). However, ADE pathology is not exclusive for
inactivated vaccines and has been also demonstrated for the vectored vaccine expressing
N protein, a replicon particle platform expressing S protein (53), the recombinant
protein S with or without gold nanoparticles (54) and a MVA vectored vaccine expressing
S proteins (53, 55).
With the aim of preventing these safety issues in SARS-CoV-2 vaccines CEPI and the
Brighton Collaboration Safety Platform for Emergency Vaccines (SPEAC) convoked an
expert scientific meeting on March 12 and 13, 2020 in order to establish the assessment
of the risk of ADE during SARS-CoV-2 vaccine development (53).
In murine models, ADE was observed for an inactivated whole virus vaccine against
MERS (56) and against SARS-CoV-1 (53, 57, 58). In fact an inactivated MERS-CoV vaccine,
with and without adjuvant, induced in mice neutralizing antibody, reduced the viral
load in lungs but showed mononuclear infiltrates containing eosinophils and eosinophils
secreting IL-5 and IL-13 cytokines (56). A formalin double-inactivated SARS-CoV-1
(DIV) vaccine, adjuvanted or not, induced also immunopathology involving eosinophils
in aged mice (53, 57). In addition, a formalin-inactivated SARS-CoV-1 vaccine promoted
ADE in NHP with macrophage and lymphocyte infiltration in the lungs and fibrin and
protein-rich edema in the alveolar cavity (58). On the other hand, other inactivated
vaccines against SARS were reported as non-inducers of ADE (59).
Fortunately, other studies disclosed the absence of ADE in hamsters and monkeys immunized
with whole inactivated vaccine against SARS-CoV-1. These studies differed from the
previous one in the use of β-Propiolactone instead of formalin to inactivate the virus
(60, 61).
The conclusion was that, NHPs could be used to evaluate the anti-COVID-19 vaccines
with or without adjuvants to select the formulations with desired efficacy and reduced
risk of ADE (53). In addition, transgenic mice expressing the human ACE receptor will
be needed to evaluate the vaccine induced ADE. The immunopathology was a consequence
to a Th2 type of response to the antigen and it was avoided in vaccines that drive
the response to a Th1 immunity, with or without adjuvants. Also, it is known that
the presence of fetal calf serum in the preclinical vaccine preparation may induce
eosinophil influx to lungs (53).
For instance, the passive transfer in NHPs of human antibodies generated during Phase
1 trials, followed by viral challenge could be considered to assess the risk of disease
enhancement (53). It was recommended to challenge the vaccinated animals with close
related species in order to evaluate cross protection for future epidemic (15, 53).
This has been done for the whole inactivated Sinovac vaccine with other isolates of
the SARS-CoV-2 (40). Experts also recommended including animals vaccinated with formalin
inactivated, alum-adjuvanted whole virus SARS-CoV-1 or SARS-CoV-2, for immunopathology
studies, as positive controls. This will help in establish accepted end-points to
allow comparison (53). The group of experts considers that continuous monitoring of
this risk will be needed during clinical trials. Each effect observed should be discussed
by the developers with their regulators who will ultimately define the actual requirements
for clinical studies (53).
If we consider that a whole virus inactivated vaccine with a potent QS21 saponin adjuvant
is the ideal formulation for an anti-COVID-19 urgent first vaccine, the Sinovac vaccine
is not only the closest formulation to the ideal, only differing in the adjuvant,
but also the one that can be developed the fastest. The potential use of alum, MF59,
AS03, AS04, or AS01, which contain QS21 saponin has been discussed (53, 62). It was
concluded that, the immunopathology of SARS vaccines was a consequence to a Th2 type
of response to the antigen and it was avoided in vaccines that drive the response
to a Th1 immunity, with or without adjuvants (53).
Time matters and is an extremely important factor considering the high daily rate
of deaths worldwide. In fact, the assays of the Sinovac vaccine in the mouse, rat,
and macaque models seems to have been performed simultaneously from January to March,
2020. In addition, this is the only vaccine with results already published in a peer
reviewed Journal (40).
On the other hand, eradication of the pandemic or at least its control, as Jenner
knew in 1796, will only be possible by a universal and simultaneous use of the same
vaccine. This is what took place with smallpox, rabies, yellow fever, Influenza, H1N1,
etc. In spite of that, we do not see the united international effort to gather together
resources for the production of enough doses of one vaccine to vaccinate the World.
The support given to 147 different research projects (36) and the deposit of hundreds
of patents confirms that. Again, the urgent formulation might already be known and
waiting to be rediscovered from the history of vaccinology. If different vaccines
with diverse degrees of efficacy values are used, even the countries that have low
incidence of COVID-19 will not be safe and will not be able to open their frontiers.
To support the production of one ideal vaccine should be the common focus worldwide.
Maybe the observed multiple individualistic efforts that have arisen are due to the
lack of leadership from the developed nations, which have the highest capacity to
produce vaccines. In the USA, for instance, the economic interest of the large vaccine
industries in preventive vaccination has recently decreased. Conversely, they have
started to invest in immunotherapies or drug treatments. In addition, in the USA there
are not large Public Laboratories for production of the vaccines under a governmental
request. Consequently, the governmental Public Health decisions are restricted by
the interests of the private vaccine companies. In contrast, in some developing countries,
where infectious diseases are often the most important causes of mortality, Public
Laboratories can produce large amounts of vaccine doses without the need to make a
profit, under the auspices of their Ministries of Health. This is the case of Instituto
Butantan and Bio-Manguinhos in Brazil, Instituto Biológico de La Plata and the Administración
Nacional de Laboratorios e Institutos de la Salud (ANLIS-Malbrán) in Buenos Aires,
Argentina, and of the Serum Institute of India. Fortunately, Instituto Butantan will
produce the Sinovac inactivated vaccine and Bio-Manguinhos the adenovirus Chadox1
vaccine of Oxford in Brazil. The Serum Institute of India will also produce the Chadox1
vaccine of Oxford.
Finally, the modern policies for vaccine regulations should be taken into consideration.
These regulations demand that Phase I, Phase II, and Phase III trials should be developed
with success before a government licenses a vaccine and uses it on Phase IV trials
and for industrialization. This usually takes at least a decade. Although these tests
enhance the confidence of a product, one might think that if we are dealing with a
vaccine produced by a technology that is already well-established in many licensed
vaccines, such as a whole inactivated virus, more rapid or simultaneous tests would
be accepted as proofs of concepts (38). This would be another increased cost-benefit
value of the vaccine, which takes into account the high mortality, worldwide incidence
and impressive impact on the economy promoted by the quarantines. Probably, this is
what Jenner and Pasteur would have done.
Author Contributions
CP-S designed and wrote this article.
Conflict of Interest
The author declares that the research was conducted in the absence of any commercial
or financial relationships that could be construed as a potential conflict of interest.