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DEVELOPMENTS ASSOCIATED WITH DENGUE VACCINE
Umar Farooq Gohar and Zahid Hussain,
Abstract
In the present era about 40% of the total world population is facing the risk of Dengue
Virus infection. Every year, 20,000 people die, mostly in tropical and subtropical regions
and the number is increasing every year. Dengue virus belongs to the family Flaviviridae
with four serotypes (recognized as DENV-1, DENV-2, DENV-3 and DENV-4) and for
the time being, more than 10,000 strains have been identified. Virus has about seven nonstructural proteins involved in viral replication within the host cell. More recently, the
application of recombinant DNA approaches to dengue vaccine development has
provided significant advances, and several vaccine candidates are progressing in clinical
trials. The most successful product among these has been Sanofi Pasteur’s ChimeriVaxDengue tetravalent vaccine which is being under trial. The ChimeriVax technology is
based on four recombinant viruses containing the prM and E genes from each of the four
dengue virus types inserted into the Yellow Fever virus (YFV) 17D vaccine strain
backbone.
Introduction
As things stand in the modern day, more than 40% of the total world population is
at risk of Dengue Virus infection. Every year, 20,000 people die, mostly in tropical and
subtropical regions and the number is increasing every year (Qi et al., 2008). Someone
may think Dengue as a relatively new disease under the lights of Malaria and other
hemorrhagic fevers but the first reported outbreak of dengue is as old as mankind
(Guzman and Istúriz 2010).
The oldest records of Dengue incidence can be found in the Chinese medical
encyclopedia for the years 265 A.D. The script describes the infection as 'poison water'
which is somehow associated with the flying insects (mosquitoes). Before its epidemics
in Asia, Africa and North America in late 1770s, the first outbreak of the illness was
reported in the Caribbean in 1635 and then in 1699 (Vasilakis and Weaver, 2008). Since
then, different outbreaks have been reported in various regions across the globe. World
War II however, played a significant role in the spread of Dengue in Asia-Pacific region
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due to high mobility of civilians and soldiers and increased numbers of susceptible
individuals in endemic areas (Gubler 2006; Kuno, 2007).
Now the question here is if the disease has been known for that long, why still we
don’t have any licensed vaccine against this infection. There have been some hurdles
which are needed to be addressed first before we can commercialize any appropriate
vaccine for all 4 serotypes of the virus.
Since the last two decades, the dengue incidences have been increased rapidly
which has brought many groups including: The Pediatric Dengue Vaccine Initiative
(funded by Bill and Melinda Gates Foundation), the WHO, the US military, as well as
industry and governments in many different countries to collaborate together in the hope
of accelerating the development of a successful vaccine (Murrel, 2011). With our better
understanding of the virus particle; several vaccine candidates are now showing promise
in clinical studies, with the most advanced candidate having entered Phase 3 testing
(Beth, 2011).
Dengue virus has 4 serotypes (recognized as DENV-1, DENV-2, DENV-3 and
DENV-4) and for the time being, more than 10,000 strains have been identified and
reported in the biological databases all over the world. Seven non-structural proteins are
also involved in viral replication within the cell, named NS1, 2A, 2B, NS3, 4A, 4B and
NS5. The E Glycoprotein is important for attachment and entry into the cells. The
proteins bind to the specific receptors on the host cell surface to ensure the entry of viral
genetic material into the host cell. The protein structure arrangement may vary slightly in
all 4 different serotypes but overall, all 4 serotypes have similar effects and functions.
The pathogenecity however, can be different for each serotype (Murrel et al., 2011).
Dengue virus is known to enter cells through receptor mediated endocytosis.
Several primary cellular receptors and low-affinity co receptors for flaviviruses have
been identified. Dendritic cell-specific ICAM-grabbing non-integrin and CD14
associated molecules have been suggested as the primary receptors for dengue virus.
Heparin and other glycosaminoglycans act as low affinity co-receptors for several
flaviviruses..
On the incidence of primary infection with DENV, the incubation period lasts for 4–7
days on average. In this time the virus replicates in the dendritic cells in close proximity
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to the bite, also infecting macrophages and lymphocytes, and then ultimately into the
bloodstream. Dendritic cells are antigen presenting cells that are integral to inducing an
immune response which are targeted preferably by the virus particle (Hung et al., 2004).
Cell surface heparin sulfates are thought to be involved in cell surface attachment
of dengue virus to mammalian cells (Chen et al., 2008). Scientists have also analyzed the
host receptors used by the virus for the attachment purpose e.g CD209/DC-SIGN (a Ctype Lectin), and is thought to bind to the viral E protein. C-type lectin domain family 5
A on the other hand can serve as a pattern recognition receptor and facilitate its entry into
the macrophages (Fried et al., 2006). After virus attaches itself to the host cell, the
replication process begins and new virus particles are released ultimately from the cell
(Fried et al., 2006).
During the secondary infections, the increase in viral replication directly increases
the amount of virus in the blood, contributing to DHF/DSS (severe forms of dengue
fever). A strong association between severe secondary infections and host genetics, as
well as virulence of different virus strains can also play part in DHF pathogenesis (Fried
et al., 2006).
Major Vaccines in Development against Dengue
Despite of a lot of research for the vaccine development, the complications posed
by the four independent virus targets and the accepted need for balanced tetravalent
responses have presented challenges that have stymied the field until recently. The
Clinical testing phase of classically attenuated vaccine candidates (e.g. passage through
cell substrates) started in the 1980s. Despite the challenges, two products were able to
reach Phase 2 clinical testing (Sun et al., 2006; Kitchener et al., 2006).
However, the inability to adequately assess attenuation and the interference
between the viruses has led to further evaluation of these candidates’ vaccines being put
on hold (Sanchez et al., 2006; Guy et al., 2010). More recently, the application of
recombinant DNA approaches to dengue vaccine development has provided significant
advances, and several vaccine candidates are progressing in clinical trials (Table 1).
The most successful product among these has been Sanofi Pasteur’s ChimeriVax
Dengue tetravalent vaccine which is being under trial in Thailand and has reached phase
3 of clinical testing (Guy et al., 2008). The ChimeriVax technology is based on four
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recombinant viruses containing the prM and E genes from each of the four dengue virus
types inserted into the Yellow Fever virus (YFV) 17D vaccine strain backbone. The
results obtained from Phase 1 and 2 testing have been quite promising and the product
induces tetravalent virus neutralizing antibody responses in 77–100% of vaccines
following administration of 3 doses of vaccine at roughly 4–6 month interval (Blaney et
al., 2006).
A major challenge however for the tetravalent ChimeriVax-Dengue product is the
extended dosing schedule (3 doses at 6 month intervals). This extended schedule is longer
than most licensed vaccine schedules and could present major issues in terms of
compliance. Furthermore, incomplete immunity over the course of 12 months may put
vaccines in endemic areas at increased risk of exacerbated disease. Thus, while the
tetravalent ChimeriVax Dengue vaccine candidate appears very promising thus far in
clinical evaluations, there may be room for improvement (Beth et al., 2011).
The National Institute of Allergy and Infectious Disease (NIAID) Laboratory of
Infectious Diseases (LID) has also been working on a live attenuated tetravalent vaccine
which uses a different strategy than that of ChimeriVax Dengue (Durbin et al., 2006).
The clinical testing has now advanced to tetravalent product testing with three different
formulations undergoing Phase I clinical testing (TV001, TV002, and TV003).
Preliminary data from these trials suggest that the tetravalent vaccine is safe and may
offer advantages in terms of balanced tetravalent responses compared to other live
attenuated viruses. The LID has licensed this deletion/chimeric technology to several
developers in dengue-endemic countries, including Instituto Butantan in Brazil,
Biological E and Panacea Biotec in India, and VaBiotech in Vietnam. These developers
are expected to move the product forward for product registration in their respective
countries (Huang et al., 2003). Other companies are also experimenting on developing a
vaccine against Dengue and most of them have entered the first phase of clinical trials
(Clements et al., 2011; Guirakhoo et al., 2006).
Challenges We Still Face
While progress towards the development of a safe and effective dengue vaccine is
significant, the key challenges remain the lack of understanding of protective immune
responses in humans as described above and the ability of vaccine candidates to induce
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long-lasting, balanced, tetravalent responses. While the initial data from the phase II
clinical trials of ChimeriVax-Dengue appear promising, a challenge for this product, and
perhaps for all live attenuated dengue virus vaccines, is the extended dosing schedule that
is required. Studies of all tetravalent live attenuated dengue vaccines conducted to date
suggest that a minimum of 4–6 months is required between doses. Thus, full tetravalent
immunity using a three-dose schedule, with doses administered at 6-month intervals may
not be achieved for 12 months. The impact of this delay on the protection of individuals
in endemic areas of dengue virus in the intervening period remains to be determined.
Thus far, based on the clinical trials and long term follow up of classically
attenuated vaccines, there have been no cases of DHF reported in trial participants in
endemic areas, despite the fact that some of the volunteers received less-than-ideal
vaccine formulations. This suggests that even vaccination with suboptimal formulations
has not resulted in a major increase in the risk for DHF in vaccine recipients.
Furthermore, vaccine induced immune responses in individuals with prior Flavivirus
exposure, as frequently occurs in endemic areas, appear to be elevated and broader than
responses in naïve individuals, as a result of the shared epitopes within the Flavivirus
genus (Guy et al., 2008; Guirakhoo et al., 2006). Such immune responses are likely to
result in more rapid and robust efficacy for vaccine recipients with prior dengue exposure
compared with naive recipients. Exposure to additional dengue antigens through natural
exposure or vaccination is also likely to contribute to durability of the immune response,
representing a ‘booster’ to an already primed immune system (Guy et al., 2008).
Conclusion
Scientists have been working to sort out the issue for a long time now and with
the increasing number of incidences and 40% of population at risk, a finalized product
seems to be a desperate need. The challenges associated with the limited understanding of
protective responses and those factors that determine disease severity still remain to be
disclosed completely. But with prospective studies ongoing in various dengue endemic
areas and the initiation of dengue vaccine trials, immune responses are being evaluated in
the context of protection and severe disease and these studies are highly likely to provide
additional insights. We are hopeful that soon, there will be a vaccine available in the
market against all 4 types of Dengue viruses.
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References
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attenuated dengue virus vaccine using reverse genetics. Viral Immunol., 19(1): 10-32.
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Table: - 1 Vaccines currently under investigation at different stages of clinical trials.
Vaccine
Vaccine Approach
Candidate
Developer/Com
Stage of Clinical
mercial Partner
Development
Chimerivax
Chimeric viruses based on Sanofi
dengue LAV
YF 17D back bone
Pasteur. Phase 3
Walter
Reed Phase
2
Classically live attenuated Army Institute of
(development on
viruses derived by
hold)
passage
in
Research/GlaxoS
primary
dog mithKline
kidney cells
TV
vaccine Combination
formulations
of
defined NIAID
Phase 1
mutations/deletions
LID/Butantan,
and chimeras
Biological E,
Panacea,
Vabiotech
DENVax
Chimeric viruses based on Inviragen
Phase 1
DENV2 PDK 53
Background
DEN-80E
Recombinant
subunit Hawaii
envelope proteins
Phase 1
Biotech/Merck
produced in insect cells
DEN-80E
Recombinant
subunit Naval
envelope proteins
Medical Phase 1
Research Center
produced in insect cells
Acronyms:
YF 17D: - Yellow Fever 17D vaccine virus strain.
LAV: - Live attenuated virus (classically attenuated vaccine strains).
TV Vaccine formulations: - Tetravalent vaccine formulations under evaluation by
NIAID.
NIAID LID: - National Institute of Allergy and Infectious Diseases, Laboratory of
Infectious Diseases.
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DENVax: - Trade name of chimeric vaccine approach being evaluated by Inviragen.
DENV2 PDK53: - Dengue virus type 2 strains passaged 53 times in primary dog kidney
cells.
DEN-80E: - Recombinant subunit corresponding to the carboxy-terminal approximately
80% of the complete dengue envelope protein.
D1ME-VR-P: - DNA construct under evaluation by the Naval Medical Research Center
that includes full-length membrane and envelope genes.
prM-E: - Pre-membrane and envelope.
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