Vaccine 34 (2016) 2948–2952
Contents lists available at ScienceDirect
Vaccine
journal homepage: www.elsevier.com/locate/vaccine
Status of vaccine research and development of vaccines for herpes
simplex virus夽
Christine Johnston a,d,∗ , Sami L. Gottlieb e , Anna Wald a,b,c,d
a
Department of Medicine, Seattle, WA, USA
Laboratory Medicine, University of Washington, Seattle, WA, USA
Seattle, WA, USA
d
Vaccine and Infectious Disease Division, Fred Hutchinson Cancer Research Center, Seattle, WA, USA
e
Department of Reproductive Health and Research, World Health Organization, Geneva, Switzerland
b
c
a r t i c l e
i n f o
Article history:
Available online 11 March 2016
Keywords:
Herpes simplex virus
Genital herpes
Vaccine
a b s t r a c t
Herpes simplex virus type-1 (HSV-1) and -2 (HSV-2) are highly prevalent global pathogens which commonly cause recurrent oral and genital ulcerations. Less common but more serious complications include
meningitis, encephalitis, neonatal infection, and keratitis. HSV-2 infection is a significant driver of the
HIV epidemic, increasing the risk of HIV acquisition 3 fold. As current control strategies for genital HSV2 infection, including antiviral therapy and condom use, are only partially effective, vaccines will be
required to reduce infection. Both preventive and therapeutic vaccines for HSV-2 are being pursued and
are in various stages of development. We will provide an overview of efforts to develop HSV-2 vaccines,
including a discussion of the clinical need for an HSV vaccine, and status of research and development
with an emphasis on recent insights from trials of vaccine candidates in clinical testing. In addition, we
will touch upon aspects of HSV vaccine development relevant to low and middle income countries.
© 2016 World Health Organization; licensee Elsevier Ltd. This is an open access article under the CC
BY license (http://creativecommons.org/licenses/by/3.0/).
1. About the disease and pathogen
Herpes simplex virus types 1 (HSV-1) and 2 (HSV-2) are responsible for recurrent, painful oral and genital ulcers, and cause
meningitis, encephalitis, neonatal infection, and keratitis. HSV
infection affects all age groups around the globe. These large DNA
viruses are members of the human herpesvirus family, which also
includes the pathogens varicella-zoster virus (VZV), Epstein–Barr
virus, cytomegalovirus, human herpesvirus-6, -7 and -8. HSV-1 and
HSV-2 are closely related, sharing >80% identity on the amino acid
level and both types are capable of infecting the oral or genital skin
or mucosa and cause recurrent ulcerations [1]. The virus infects
epithelial cells at skin or mucosal surfaces, and then infects nerve
endings, traveling via retrograde transport to the nerve axon, where
it establishes persistent infection in the trigeminal or lumbosacral
夽 This is an Open Access article published under the CC BY 3.0 IGO license which
permits unrestricted use, distribution, and reproduction in any medium, provided
the original work is properly cited. In any use of this article, there should be no
suggestion that WHO endorses any specific organisation, products or services. The
use of the WHO logo is not permitted. This notice should be preserved along with
the article’s original URL.
∗ Corresponding author at: University of Washington, Box 359928, 325 Ninth Ave,
Seattle, WA 98104, USA.
E-mail address: [email protected] (C. Johnston).
ganglia. The virus returns to epithelial surfaces via the axon to
cause oral or genital ulcers or frequent asymptomatic viral shedding. The ability of the virus to be acquired and transmitted in the
absence of symptoms allows it to spread efficiently throughout the
population. As most infections are subclinical, disease incidence
and prevalence data underestimate the impact of HSV infection.
Although the clinical manifestations of infection are similar
between HSV-1 and HSV-2, the age groups affected and severity
of infection are influenced by the infecting virus type, the portal of
entry, host immune status and whether the infection is initial or
recurrent. HSV-1 has been more traditionally associated with oralfacial infections, although it is now a leading cause of first episode
genital herpes and neonatal herpes in high-income countries (HIC)
[2,3], which is likely related to declining HSV-1 acquisition during
childhood in these settings [4]. In LMIC, HSV-1 is rapidly acquired
during childhood, with more than 90% people infected by adolescence [5]. HSV-1 causes oral ulcers of varying severity, from
herpes labialis to gingivostomatitis and pharyngitis. It is the leading cause of sporadic infectious encephalitis (HSV encephalitis) and
infectious blindness (HSV keratitis) in HIC; the burden of these
complications of HSV-1 infection in LMIC is unknown but is likely
to be high.
Herpes simplex virus type 2 (HSV-2) is a sexually transmitted infection that is the leading cause of genital ulcer disease
(GUD) worldwide [6,7]; HSV-2 also causes neonatal herpes and
http://dx.doi.org/10.1016/j.vaccine.2015.12.076
0264-410X/© 2016 World Health Organization; licensee Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/).
C. Johnston et al. / Vaccine 34 (2016) 2948–2952
increases the risk of acquiring HIV infection. An estimated 417 million persons aged 15–49 years old are infected with HSV-2, with
an incidence of 19 million infections per year [8]. HSV-2 is rapidly
acquired among men and women initiating sexual activity in settings with high HSV-2 seroprevalence. For instance, in sub-Saharan
Africa, the incidence among women is up to 23 per 100 person
years and among men is up to 12 per 100 person years [9]. Neonatal herpes incidence in HIC is ∼10 cases/100,000 live births [10];
the incidence in LMIC is unknown. Although rare, neonatal herpes
is associated with high morbidity and mortality and no prevention strategies have been identified. HSV-2 fuels the HIV epidemic
by increasing the risk of HIV acquisition 3-fold [11], likely through
HSV-2 associated genital tract inflammation [12,13]. In addition,
genital ulcer disease increases the risk of HIV transmission [14]. In
settings with high HSV-2 prevalence, an estimated 25–50% of HIV
infections are attributable to HSV-2 [15,16].
Global estimates for the economic burden of HSV infection are
not available. In the United States, HSV-2 infection is estimated to
have a total lifetime cost of $540 million US dollars (adjusted to
2010 dollars), behind only HIV and HPV in costs among 8 major
STIs [17]. These cost estimates exclude neonatal herpes, which
had a total hospitalization charge of $35 million in 2006 in the
United States [10]. These costs also exclude sequelae of HSV-1 infection and the contribution of HSV-2 infection to HIV susceptibility.
The updated Global Burden of Disease Study (GBD) estimates that
genital HSV resulted in 311,600 years lived with disability (YLD)
in 2013 (95% uncertainty interval 98,300–748,500) due to genital ulcer disease alone [18]. GBD 2013 likely underestimates the
impact of genital HSV, as these YLD estimates do not include disability due to neonatal herpes nor the contribution of genital HSV
to HIV susceptibility, which are the most devastating consequences
of infection.
The diagnosis of HSV infection may be made by demonstrating
the virus is present in oral and genital lesions, using polymerase
chain reaction assays, viral culture or antigen detection. In addition,
commercially available “type-specific” serologic assays based on
the glycoprotein surface proteins can differentiate between HSV1 and HSV-2 infection. Serologic assays for HSV-2 are limited by
low positive predictive value in low prevalence settings and false
positive results in some populations [19].
Although HSV clinical recurrences are commonly self-limited,
some complications can be life-threatening, particularly in those
with immature or compromised immune systems. Treatment
strategies for genital herpes are limited to the antiviral agents,
acyclovir, valacyclovir, or famciclovir [20]. HSV genital ulcer disease can be treated by the antiviral agent acyclovir, which is now
included in WHO guidelines for syndromic management of GUD
[21]. While episodic therapy for genital ulcers shortens the duration of the ulcer by approximately 1 day, daily suppressive therapy
prevents most recurrences. Although daily suppressive therapy for
HSV-2 infected persons decreases the risk of transmission by ∼50%
in discordant heterosexual partnerships in the US [22], such therapy was not effective for prevention of HSV-2 transmission from
HIV-1/HSV-2 co-infected adults to HIV-1/HSV-2 seronegative partners in discordant heterosexual African couples [23]. In addition,
suppressive antiviral therapy for HSV does not reduce the risk of
HIV-1 acquisition or HIV-1 transmission [24,25]. Condoms have
partial efficacy in preventing acquisition of genital herpes [26].
2. Overview of current efforts
2.1. Biological feasibility for vaccine development
There are no vaccines currently available for HSV infection, but
the pipeline is rich with candidates in various phases of development. Vaccines are currently being developed both to prevent
2949
HSV-2 infection (preventive) and to treat HSV-2 infection (therapeutic). While most HSV vaccine research has prioritized HSV-2
rather than HSV-1, HSV-2 vaccines may also have benefits in preventing or treating HSV-1 infection given the homology between
these viruses.
There are several lines of evidence that an HSV vaccine is feasible:
1. There is a safe and efficacious vaccine for varicella zoster
virus (VZV), a closely related alpha-herpesvirus. Both a liveattenuated vaccine for preventive (prevention of varicella
“chicken pox”) and therapeutic (prevention of herpes zoster or
“shingles”) indications have been developed [27]. In addition, a
subunit vaccine with a novel adjuvant was shown to prevent
herpes zoster with 97% efficacy in a recent Phase III clinical trial
[28].
2. Whether protective genital mucosal immunity could be induced
by an intramuscular (IM) vaccination has been a concerning
unknown for the HSV vaccine field. The development of the
human papillomavirus (HPV) vaccine provides ample proof of
concept that an IM vaccine can be highly efficacious against a
mucosal genital viral pathogens [29].
3. The Herpevac trial, which tested a truncated glycoprotein D2
(gD2t) vaccine in >8000 HSV-1/HSV-2 seronegative women
showed 58% vaccine efficacy for prevention of genital HSV-1
disease and 32% efficacy for prevention of HSV-1 infection [30].
The vaccine did not prevent HSV-2 disease or infection. Titers
of antibodies to gD2t were identified as a correlate of HSV-1
protection, based on increasing vaccine efficacy with increasing
titers of gD2t [31]. Further studies showed that sera of vaccinees neutralized HSV-1 3-fold better than HSV-2, suggesting that
vaccine-induced antibodies to gD2 were sufficient to prevent
HSV-1 but not HSV-2 infection. These are the first data suggesting that antibody titers are a correlate of anti-HSV-1 immunity
and provide a benchmark for inducing protective immunity.
4. Successful vaccination based eradication campaigns have been
implemented for alphaherpesviruses which infect animals, such
as bovine herpesvirus-1 and suid herpesvirus-1 (pseudorabies
virus) [32,33].
The recent availability of full-length viral sequences from
around the world for both HSV-1 and HSV-2, will allow selection
of vaccine targets that could potentially provide protection against
both pathogens and targets that are not geographically restricted
[34–36]. This combined with increasing knowledge regarding the
role of neutralizing antibodies and T-cell responses in preventing
infection or modulating recurrent disease could help improve the
design of future candidate vaccines.
2.2. General approaches to vaccine development for this disease
for low and middle income country markets
Two approaches are being pursued for HSV-2 vaccine development: preventive vaccines and therapeutic vaccines. Preventive
vaccines would provide protective immunity against genital HSV2 infection prior to exposure, with a possible secondary effect
of prevention of HIV infection in high risk populations. The target population would be adolescent men and women prior to the
initiation of sexual activity. Prior studies of preventive HSV vaccines have focused on HSV-2 discordant couples or HSV-1/HSV-2
seronegative women in HIC; these studies have been limited by
low numbers of study endpoints (acquisition of genital herpes disease), resulting in large sample sizes and prolonged Phase III trials.
Preventive vaccines have not been tested in LMIC. However, given
the disproportionate burden of both HSV-2 and HIV infection in
LMIC, an HSV-2 vaccine must be effective in these populations to
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C. Johnston et al. / Vaccine 34 (2016) 2948–2952
have a global impact. HSV vaccines that will be tested in LMIC
must be designed to have efficacy in both HSV-1 seropositive and
HSV-1 seronegative persons, and geographic strain diversity must
be accounted for in vaccine design. In addition, testing of preventive vaccines in LMIC where HSV-2 is rapidly acquired among young
adults will allow for more efficient trials. If a candidate vaccine were
found to protect against HSV-1 as well as HSV-2 in adolescents in
HIC, shifting the timing of vaccination to infancy/childhood with
possible booster in adolescence could also be considered. Such a
vaccine may also prevent HSV-1 related eye and neurologic disease.
These issues do not have clear consensus in the field.
Therapeutic vaccines are being tested in HSV-2 seropositive persons to reduce genital lesions and genital shedding, which may
provide both personal and public health benefit. The target population for these vaccines is persons who already have recurrent
genital HSV-2 infection. Phase I/II studies of 2 candidate therapeutic vaccines have demonstrated antiviral effects, with decreased
rate of shedding and lesions and decreased quantity of virus shed
[37]. Mathematical models suggest that there is a genital viral load
threshold associated with sexual transmission [38]. Therapeutic
vaccines are being tested in HIC at this time, and there is no consensus regarding prioritization of testing these vaccines in LMIC.
An essential issue that will need to be addressed is the effect of
therapeutic vaccines on genital inflammation. A therapeutic HSV-2
vaccine could reduce genital inflammation if viral shedding is contained; alternatively, therapeutic vaccines could result in increased
genital inflammation as additional T-cells traffic to the genital tract,
theoretically further increasing the risk of acquiring HIV.
3. Technical and regulatory assessment
Preclinical development of HSV vaccines utilizes wellestablished animal models to screen and test promising candidate
vaccines. However, HSV is a uniquely human pathogen, and the
host-virus interactions differ in people, likely accounting for
limited predicted value for efficacy observed in animal models. The
mouse model is convenient due to the availability of many tools to
dissect the immune response, but has limitations, including lack of
genital reactivation and high mortality in initial infection [39]. The
guinea pig model appears to mimic human HSV-2 infection better
than the mouse model as spontaneous reactivation in the genital
track occurs [40]. However, promising vaccines in the guinea pig
model have not translated to efficacy in humans. There is often a
long delay in testing vaccine candidates with efficacy in animal
models in clinical trials.
For therapeutic vaccines in early stages of clinical development, HSV genital shedding has recently been used as a surrogate
endpoint rather than the more traditional endpoint of recurrence
frequency. Genital shedding has the potential to more precisely
define the most effective vaccine dose rather than recurrences,
which may not be recognized by participants [41]. In addition, the
use of a cross-over design in which the effect of the intervention can
be directly measured allows for efficient trial design with a limited
follow-up period and a relatively small number of participants.
4. Status of vaccine R&D activities
Multiple vaccine candidates with diverse platforms have been
studied in the preclinical phase and several are being tested in
clinical trials, with early development supported mainly by academic institutions, government, and biotech companies. Several
pharmaceutical companies have also been involved with testing
HSV vaccines.
The most widely used product for HSV vaccines in human clinical trials has been glycoprotein subunit vaccines. Glycoprotein D is
expressed on the viral surface and responsible for most neutralizing antibody activity and, therefore, is a rationale target [42].
The largest clinical trial of an HSV subunit vaccine, Herpevac,
enrolled HSV-1/HSV-2 seronegative women and used glycoprotein
D-2 (gD2) with alum/MPL adjuvant [30]. Although the Herpevac
trial did not show efficacy against HSV-2 disease, the finding that
the vaccine prevented genital HSV-1 disease (vaccine efficacy = 58%,
95% CI 12–80%) is a significant advance for the HSV field [30]. In
addition, identification that the gD2 antibody concentrations correlated with protection against HSV-1 infection, provides proof
of concept that protective mucosal immunity can be stimulated
Table 1
Development Status of Current Vaccine Candidates.
Candidate name/identifier
Pharmaceutical
developer
Platform/antigens
GEN-003
HerpV
Genocea
Agenus
Codon optimized
polynucleotide vaccine
VCL-HB01/HM01
HSV529
Admedus
Subunit vaccine: gD2/ICP4 with Matrix M2 adjuvant
32 35-mer peptides, complexed with HSP, QS-21
adjuvant
DNA vaccine: gD2 codon optimized/ubiquitin-tagged
gD2/gC2/gE2
HSV-2 0NLS
HF10
gD2
AD472
CJ2-gD2
Prime-pull strategy
Inactivated HSV-2 in
MPL/alum
HSV-1 glycoprotein B
lentiviral vector
gB1s-NISV
Vical
Sanofi
DNA vaccine: gD2+/−UL46/Vaxfectin
Replication-defective HSV-2 with deletions of UL5 and
UL29
Subunit vaccine: gD2/gC2/gE2
Live, attenuated replication-competent HSV-2 with
deletion of ICP0
Live, attenuated replication-competent HSV-1 mutated
for UL43, UL49.5, UL55, UL56, LAT
Live, attenuated HSV-2 deleted in gD2
HSV-2 mutated for g34.5, UL43.5, UL55-56, US10,
US11, US12
Non-replicating gD2 dominant neg HSV-2
“Prime” with live attenuated HSV-2 followed by “pull”
with topical intravaginal CXCL9/CXCL10 chemokine
Formalin inactivated HSV-2
Pre-clinical
Phase I
X (P&T)
Phase II
Phase III
Refs.
X (T)
X (T)
[37,51,52]
[53]
X (T)
[54]
X (T)
[55]
[56]
X
X
[49]
[57]
X
[58]
X
X
[44]
[59]
X
X
[60]
[48]
X
[61]
Lentiviral vector expressing gB1
X
[62]
Intranasal non-ionic surfactant vesicles containing
recombinant HSV-1 gB
X
[63]
gD, glycoprotein D; ICP, infected cell protein; HSP, Heat shock protein; UL, unique long; gC, glycoprotein C; gE, glycoprotein E; US, unique short; TK, thymidine kinase; MPL,
monophosphoryl lipid A; gB, glycoprotein B; T, therapeutic; P, preventive.
C. Johnston et al. / Vaccine 34 (2016) 2948–2952
through vaccination [31], although it is not clear whether this is
a mechanistic correlate [43]. Second generation preventive vaccines may need to stimulate higher titers of neutralizing antibody
or induce other immune responses for more complete protection
against HSV-1 or HSV-2.
There are several live-attenuated or replication-defective virus
vaccine candidates in the preclinical phase (Table 1). A replicationdefective HSV-2 vaccine (HSV529) has entered Phase I trials for
both preventive and therapeutic indications. A live-attenuated
virus deleted in gD2 prevented skin, neural and vaginal disease
in the mouse model, and also is the first construct to eliminate
establishment of latency in the dorsal root ganglia [44]. Concerns
about recombination with wild type virus and retained pathogenic
potential (especially with respect to central nervous system invasion) have limited the investigations into live-attenuated candidate
products [45].
Within the past 2 years, 4 additional candidates have entered
into Phase I/II trials as therapeutic vaccines. These vaccine candidates have novel adjuvants which stimulate T cell immunity.
Preliminary results were reported for GEN-003, a gD2/ICP4 protein
subunit vaccine with Matrix M adjuvant, with ∼50% decline in genital HSV shedding rate after the therapeutic vaccine series [37]. This
candidate vaccine is currently in a Phase II dose-ranging safety and
efficacy trial. Preliminary results from a Phase II study of HerpV,
a peptide vaccine with 32 peptides linked to heat shock protein
(HSP) and QS-21 adjuvant, showed a 15% decrease in viral shedding, which persisted up to 6 months after the initial vaccine series
[46]. A DNA vaccine with gD/UL46, adjuvanted with Vaxfectin® has
also entered Phase I/II trials for a therapeutic indication, with initial
results expected in mid-2015.
The realization that genital HSV infection induces tissue resident memory T cells in human genital mucosa has been a recent
advance in the field [47]. Animal studies have demonstrated the
importance of stimulating tissue resident memory T-cells for prevention of HSV infection in the mouse model using a “prime and
pull” approach, in which a topical chemokine applied to the genital
mucosa after subcutaneous vaccination drew HSV specific CD8+ T
cells and was associated with decreased clinical disease upon challenge with HSV-2 [48]. While this approach has not entered clinical
trials, it is an innovative and highlights the importance of tissue
resident T cells in the genital tract.
Novel delivery methods of glycoproteins, including lentiviral
vectors expressing glycoprotein B and intranasal delivery, are being
explored. Glycoprotein candidates with novel platforms are still
being investigated. For instance, a gD/gC/gE subunit glycoprotein
candidate is promising in mice [49].
5. Likelihood for financing
As HSV-2 infection greatly enhances the risk of HIV acquisition
and transmission, it is possible that prevention of HSV-2 infection with a vaccine would have a substantial impact on reducing
incident HIV infections, as predicted by modeling studies [50]. In
this case, the vaccine may be supported in part by the Global Fund
to Fight AIDS, Tuberculosis and Malaria in addition to GAVI. New
global estimates for neonatal herpes, which are currently being
completed, and better primary data on neonatal herpes incidence
in LMIC will additionally help inform the likelihood of GAVI prioritization based on under 5 lives saved in GAVI eligible countries.
Although neonatal HSV is relatively rare in HIC, there are few
data regarding incidence in LMIC. This condition has a 60% fatality rate without treatment and can result in lifelong neurological
deficits, even with successful treatment. Although concerns have
been raised in some settings about providing an STI vaccine to preadolescents, GAVI support for HPV vaccine offers a clear precedent
2951
for GAVI prioritization of an adolescent vaccine against a sexually
transmitted infection.
Conflict of interest: The University of Washington has received
funds to conduct research sponsored by Agenus (AW), Genocea
(AW), Vical (AW), Genentech (AW), Gilead (AW) AiCuris (CJ), and
Sanofi (CJ). AW also serves as a consultant to Amgen and AiCuris.
Funding source: This work was supported by National Institutes
of Health grants (PO1 AI030731) to CJ and AW.
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