51 years in of Chikungunya clinical vaccine development: A historical perspective

ABSTRACT

Chikungunya fever (CHIKF) is a mosquito-borne disease caused by Chikungunya virus (CHIKV). This virus is considered a priority pathogen to the UK government, the US National Institute of Allergy and Infectious Diseases (NIAID) and the US military personnel, due to the potential of CHIKV to cause major outbreaks. Nearly all CHIKV infections are symptomatic, often incapacitating and patients experience severe joint pain and inflammation that can last for more than one year with 0.4–0.5% fatality rates. Mother-to-child transmission has also been described. Despite this re-emerging disease has been documented in more than 100 countries in Europe, Oceania, Africa, Asia, the Caribbean, South and North America, no licensed vaccine is yet available to prevent CHIKF. Nevertheless, various developments have entered phase I and II trials and are now viable options to fight this incapacitating disease. This review focuses on the development of CHIKV vaccines that have reached the stage of clinical trials since the late 1960s up until 2018.

Introduction

Chikungunya Virus (CHIKV) is an arthropod-borne virus (arbovirus) transmitted to humans by the Aedes aegypti and A. albopictus mosquitoes. It is a member of the alphavirus genus, part of the Togaviridae family. The alphavirus genus comprises 29 species classified into seven antigenic complexes that include Eastern equine encephalitis (EEE), Western equine encephalitis (WEE), Venezuelan equine encephalitis (VEE), Barmah Forest (BF), Middelburg (MID), Ndumu (NDU) and Semliki Forest (SF), with CHIKV being a member of the latter complex.¹

Alphaviruses have a wide geographic distribution. Infection in humans leads to two major clinical outcomes, (A) arthralgia and arthritis caused by Old World alphaviruses such as CHIKV, O’Nyong Nyong and SF viruses and (B) encephalitis, caused by the New World alphaviruses such as VEE and WEE viruses.² Vaccines against alphavirus infections have been under development for several years and while major progress has been made for CHIKV prompting its assessment in clinical trials, most alphavirus vaccines remain in pre-clinical stage in mouse and macaque models. Nevertheless, CHIKV vaccines form a robust pipeline with promising developments from pre-clinical to phase I and II clinical trials, which have been thoroughly reviewed before.^3,4 A summary of the past and current developments shows that while most vaccine candidates are in early mouse and macaque pre-clinical phases, a good number are transitioning through phase I and II trials with promising results (Figure 1).

Figure 1.

Summary of past and current CHIKV vaccines assessed in early pre-clinical phase or late phase I and II clinical trials.

Chikungunya virus (CHIKV) is one of the simplest enveloped viruses. It is a small and spherical virus of an approximate size of 60–70 nm.^1,5 The genomic organisation consists of a single-stranded positive-sense RNA of approximately 11.8 kb in size that resembles the eukaryotic mRNA due to the presence of 5ʹ cap structures and a 3ʹ poly (A) tail.¹ The genome size varies geographically among lineages, being longest in the Asian strains (11,777–11,999 nucleotides or nt), followed by West African (11,843–11,881 nt) and East/Central/South African (11,557–11,789).⁶ The genome is organised into two open reading frames, one of approximately 7.4 Kb in size encoding 4 non-structural proteins (nsP1, nsP2, nsP3 and nsP4) and one of 3.7 Kb encoding the structural proteins of the virus, including Capsid (C), peptide E3, envelope glycoprotein E2, peptide 6K/TF and envelope protein E1.^1,3

Sequencing of the complete CHIKV genome has revealed the existence of four lineages: (1) West African (Waf); (2) East/Central/South African (ECSA); (3) Asian and (4) Indian Ocean Lineage (IOL).⁶ Whole genome sequencing has proven to be key to support identifying the IOL, since only three lineages had been identified using partial sequencing of the E1 gene in earlier studies that reported three lineages, thus underlying the inadequacy of the E gene to resolve the phylogenetic history of the CHIKV.⁶ These studies have been useful to trace the existence of a CHIKV ancestor within the last 500 years with indication of a divergence between the ECSA and Asian lineages occurring in the last 150 years.⁶ Phylogenetic studies indicate that the major Chikungunya Fever (CHIKF) outbreak in the Americas was caused by an Asian genotype, with the exception of Brazil where both ECSA and Asian lineages co-circulate, the latter has formed an Asian/American lineage defined by two amino acid substitutions in the E2 envelope glycoprotein and the 6K peptide region (E2 V368A and 6K L20M), which is evolving at a mean rate of 5 × 10⁴ substitutions per site per year and is similar to that previously calculated for the Asian genotype.⁷

History of CHIKV vaccines

Formalin-inactivated vaccines (FIV)

The history of CHIKV vaccines started with a formalin-inactivated approach in the late 60s (Figure 2) at the Walter Reed Army Institute of Research (WRAIR) in Washington D. C., when V.R Harrison, L.N. Binn and R. Randall did seminal work by assessing the immune responses elicited in mice and rhesus macaques by a formalin-inactivated CHIKV vaccine.⁸ The virus used to develop the vaccine was the African CHIKV strain 168, isolated from Southern Tanganyika (nowadays Tanzania) between 1952–1953.⁹ The experimental vaccines were prepared using chik-embryo (CE), suckling-mouse-brain (SMB) and green monkey kidney cells (GMKC). Poor immunogenicity was induced by the CE preparation, which contrasted with good immune responses induced by the SMB and GMKC preparations. The latter was selected for further development as vaccine from SMB products pose the risk of inducing encephalitis in man, thus limiting its applications. The ability of the CHIKV 168 vaccine to elicit homologous protection in mice prompted researchers to assess heterologous protection in rhesus macaques using various CHIKV strains: the African CHIKV strain 168; the CHIKV strain E.103 isolated from a pool of 78 Ae. Africanus mosquitoes at the Zika forest on June 12, 1956 by M. C. Williams;¹⁰ the Asian strain BAH-306 isolated from Thai patients¹¹ and the Indian CHIKV strain C-266 isolated from Calcutta by K.V. Shah.¹² Vaccination of eight rhesus macaques consisted of three doses of 1 ml each of GMKC-prepared CHIKV strain 168 (harvested after 72 h of infection with 10⁵ SMICLD₅₀) administered subcutaneously on day 0, 7 and 21. Homologous (strain 168) and heterologous challenges (strains E.103, BAH-306, C-266, mentioned above) were done 30 days after vaccination. Results yielded complete absence of viremia upon either, homologous or heterologous challenge in all vaccinated monkeys. This was the first demonstration of the homologous and heterologous protective efficacy by a formalin-inactivated CHIKV vaccine in pre-clinical models using mice and rhesus macaques.⁸

Figure 2.

Timeline displaying the development of CHIKV vaccines that have entered clinical trials.

First CHIKV clinical trial

Four years later in 1971 at the WRAIR, V. R. Harrison, K. H. Eckels, P. J. Bartelloni and C. Hampton described the preparation of a formalin-inactivated CHIKV vaccine and its assessment in a phase I clinical trial (Figure 2).¹³ The vaccine seed virus was initiated from Thai human samples from the United States Army Medical Component SEATO in Bangkok, Thailand (nowadays AFRIMS), provided by Dr. E. L. Buescher. Four CHIKV isolates, designated 6348, 6461, 15561 and 23337 were grown in green monkey kidney tissue culture cells (GMKC) for 10 passages to eliminate any potential hepatitis and other adventitious viruses. Potency assays based on a mouse challenge with the CHIKV 168⁸ resulted in the selection of the CHIKV 15562 isolate (Table 1). A phase I clinical trial to assess the vaccine in sixteen young volunters of 21–25 years of age was performed. Two groups of eight volunteers were administered the vaccine subcutaneously (s.c.). Group I received two doses of 0.5 ml of vaccine at an interval of 28 days and group II received two doses of 1 ml under the same protocol. Local and systemic reactions were assessed for 12 days in a closed research ward. Thereafter, volunteers were observed on an outpatient basis and total absence of untoward reactions after the vaccination was observed. Most subjects developed neutralising antibodies on day 14 after the first vaccination and the second administration prompted the development of significant neutralising antibody titres with values of log₁₀ neutralization indices of 2.7 for either dose, thus concluding that two 0.5 ml vaccine doses administered 28 days apart are sufficient to elicit good antibody responses.¹³

Table 1.

Characteristics of Chikungunya vaccines that have entered clinical trials.

Vaccine	Platform	Lineage	CHIKV Immunogen	Adjuvant	Number of Doses (interval)	Vaccination Route	Stage of Development
CHIK Vaccine FIV 15562	Formalin-Inactivated Virus (FIV)	Asian Strain 15,561	Whole inactivated virus	No	2 (28 days)	Subcutaneous	Phase I Trial
CHIKV TSI-GSD-218	Live Attenuated Virus (LAV)	Asian Strain 15,561	Whole attenuated virus	No	1	Subcutaneous	Phase II Trial
VRC-CHKVLP059-00-VP	Virus-Like Particle (VLP)	West African (Waf) Strain 37,997	Capsid E3 E2 6K E1	No	3 (0, 4, 20 weeks)	Intramuscular	Phase II Trial
MV-CHIK	Measles Viral Vectored Vaccine (VVV)	Indian Ocean Lineage (IOL)	Capsid E3 E2 6K E1	No	2 (28 days)	Intramuscular	Phase II Trial
ChAdOx1-Chik	Chimpanzee Adenoviral Vector (ChAdOx1)	Consensus sequence from multiple lineages	Capsid E3 E2 6K E1	No	1	Intramuscular	Phase I Trial

CHIKV live-attenuated vaccines (LAV)

Production of formalin-inactivated CHIKV vaccines generates cost and safety concerns due to the requirement of BSL-3 facilities and the associated risks posed by producing large amounts of infectious CHIKV prior to being subjected to the inactivation process.¹⁴ These concerns prompted the development in the 1980s of a Live-attenuated CHIKV vaccine by scientists at the U. S. Army Medical Research Institute of Infectious Diseases (USAMRIID) in Maryland.¹⁵

Studies with the formalin-inactivated CHIKV 15561 generated extensive knowledge on the vaccine production and assessment in pre-clinical and clinical trials.¹³ Hence, the vaccine produced after 10 passages was subjected to an 11^th passage in GMKC to be subsequently transferred from WRAIR to USARMRIID to proceed to attenuation by culturing virus in human embryonic lung MRC-5 cells. 347 plaques were subjected to 18 passages in MRC-5 cells. Clone 181 produced small, 2–3 mm homogeneous plaques and the CHIK 181/clone 25 was finally chosen as the vaccine seed for subsequent vaccine production. The selected clone provided complete protective efficacy to weanling mice using an intracerebral (i.c.) challenge with CHIKV. Following these results, rhesus macaques were administered the vaccine subcutaneously and challenged with CHIKV approximately 5 weeks after vaccination. All vaccinated macaques had complete, sterile protection (Table 1).¹⁵ Induction of neutralizing antibodies was evident and peaked at day 14, with an average of PRNT₈₀ titers of 165, 440 and 800 following vaccinations with doses of 3.5, 4.5 and 5.5 log₁₀ pfu of immunizing dose.

Following these results, the CHIK 181/clone 25 was manufactured at The Salk Institute-Government Services Division (TSI-GSD) and an investigational new drug application was filed by the U.S. Department of Defense with the FDA in 1986.¹⁴ The CHIKV TSI-GSD-218 vaccine entered clinical trials at both, USAMRIID and the University of Maryland Center for Vaccine development (Figure 2),¹⁶ showing safety and immunogenicity in a phase I trial in 15 volunteers. The vaccine was subsequently tested in a phase II, randomized, double blind, placebo controlled in 59 healthy volunteers (35 men, 24 women) receiving a single s.c. vaccine dose of 0.5 ml of the reconstituted vaccine, containing 10⁵ pfu/dose. 14 volunteers (12 men, 2 women) received placebo vaccine.¹⁷ Vaccination resulted in 98% of volunteers developing neutralising antibodies and importantly, 5 (8%) CHIKV vaccinees developed transient arthralgias that were absent in the placebo group. The authors noted that the incidence of arthralgia in vaccinees compared to controls would have reached statistical significance in a study with larger vaccine and control groups.¹⁷

Developing a CHIKV vaccine with the traditional methodologies using inactivated or live-attenuated viruses present some challenges, as mentioned earlier, due to the production of large amounts of virus requiring the appropriate biosafety levels to prevent accidents before achieving inactivation, or induction of adverse events like arthralgia upon vaccination with live-attenuated viruses. Therefore, the field of CHIKV vaccine development presents opportunities for the development of new sub-unit vaccines, Virus-Like Particles, replication-defficient viral vectors, DNA vaccines and proteins that are considered safe options, albeit potentially less immunogenic thus requiring the use of adjuvants or the administration of multiple doses to enhance immunity against CHIKV. Intermitent, but major outbreaks contributed to maintaining the interests in the development of a CHIKV vaccine, prompting efforts to assess new vaccine platforms.

Virus-like particles (vlps)

VLPs are produced by the expression of viral structural proteins that self-assemble to produce structures similar to the original virus but lacking the capacity to infect and replicate. By mimicking the virus structure without the viral genomes, they resemble ‘empty shells’.¹⁸

The Vaccine Research Center (VRC) at NIH’s National Institute of Allergy and Infectious Diseases (NIAID) in Bethesda, has developed a CHIKV VLP. The vaccine is composed of the structural CHIKV proteins Capsid, E3, E2, 6K and E1 sequences of the CHIKV strain 37,997 from a CHIKV Waf lineage. The VLP vaccine has been tested in macaques (n = 6), using intramuscular regimes consisting of 20 μg of VLPs in 1mL PBS at weeks 0, 4 and 24. Immune responses were evident after a primary immunisation and increased after subsequent boosts at 4 and 24 weeks. All the immunised macaques were able to control viraemia and inflammation after a challenge with 10¹⁰ PFU of CHIKV (strain LR2006 opy-1) doses through the induction of neutralising antibodies.¹⁹

The VLP, known as VRC-CHKVLP059-00-VP was GMP produced by transfection of VRC293 cells (suspension-adapted, serum-free HEK293 cells) with plasmid DNA expressing the CHIKV structural genes (Table 1). The VLP has been assessed in the VRC 311 phase I clinical trial in a dose-escalation, open label trial to evaluate safety, tolerability and immunogenicity of the CHIKV VLP in 25 adults of 18 to 50 years of age.²⁰ The vaccine was administered intramuscularly (i.m.) to humans at doses of 10 μg, 20 μg and 40 μg given at weeks 0, 4 and 20 weeks with no adjuvant included. The vaccine was well tolerated and no serious adverse events were reported. Antibody responses by ELISA were positive upon measurement of an endpoint ELISA titer technique agains the CHIKV VLP antigen strain 37,997, reaching an average of 4,457, 5,881 and 8,611 (after receiving 10, 20 and 40 μg of VLP, respectively) on week 44 after receiving three vaccinations. Most participants showed induction of neutralising antibodies after the first VLP vaccination and all participants had neutralising antibodies 4 weeks after the second vaccination, reaching titers of 188, 236 and 346 elicited by 10, 20 and 40 μg of VLP, respectively.²⁰ The VRC-CHKVLP059-00-VP has entered phase II trials in 2015 in a multicenter study to evaluate safety and immunogenicity using two vaccine doses in 400 healthy adults between 18 to 60 years of age in locations including Dominican Republic, Guadeloupe, Haiti, Martinique and Puerto Rico ClinicalTrials.gov (NCT02562482).

Viral-vectored vaccines (VVV)

Measles viral vector

A member of the Morbillivirus genus within the family of Paramixovirus, Measles virus (MV) has been developed as a viral vectored vaccine for various diseases at the Institut Pasteur in Paris in a seminal work lead by the group of Frédéric Tangy.^21–25

In 2013, Samantha Brandler et al. reported the development of a recombinant Measles viral-vectored (MVV) vaccine expressing the heterologous structural genes of CHIKV (Figure 2).²⁶ The cDNA CHIKV structural cassette encodes the C-E3-E2-6K-E1 which in turn forms Virus-Like Particles upon expression in cell culture.²⁶ The CHIKV sequence corresponded to an Indian Ocean Lineage (IOL) isolate from a patient sampled on 2 December 2005 in the Southern locality of St. Louis, La Réunion (CHIKV strain 06–49, GenBank accession no. AM258994).²⁷

Immune responses have been evaluated in genetically modified mice expressing the human MV receptor hCD46 and no IFN-α/β receptor (CD46-IFNAR) to assess vaccine efficacy in a mouse model highly susceptible to CHIKV infection. Antibody responses were assessed after a single or a double injection one month apart with 10³, 10⁴ or 10⁵ MV TCID₅₀. A single immunisation gave ELISA endpoint titres between of 1,350, 4,050 and 12,150 that increased after a boost to 2,700, 12,150 or 48,600 for each respective MV dose. Neutralising antibodies were quantified using a plaque reduction neutralisation test (PRNT) to calculate reduction of plaque number in at least 50% (PRNT₅₀) or 90% (PRNT₉₀). PRNT₅₀ titres after a single vaccination were of 50, 150 and 450 for each increasing MV-CHIK priming dose and 450, 1,350 and 4,050 after a boost with 10³, 10⁴ or 10⁵ MV TCID₅₀. Cellular responses quantified by an ex vivo IFN-gamma ELISPOT yielded responses of a mean of 150 sfu/10⁶ splenocytes after a single immunisation with a higher MV dose of 10₆ TCID₅₀. Survival of mice after a challenge with 100 PFU of CHIKV-06–49 showed protection of 83% of the mice vaccinated with 10³ TCID₅₀, while complete protection was achieved with prime/boost using the two highest doses or upon a single prime with 10⁵ TCID₅₀, thus demonstrating the potential of the MV-CHIK as a vaccine for CHIKV and other arboviruses²⁶ and prompting its assessment in a phase I trial that was reported two years later.

The MV-CHIKV was assessed for safety and immunogenicity in a phase I clinical trial (Table 1 and Figure 2),²⁸ which enrolled 42 participants with 12 volunteers each group to receive a low (1.5x10⁴ TCID₅₀), intermediate (7.5x10⁴ TCID₅₀) or high (3.0x10⁵ TCID₅₀) dose of the vaccine and one control group (n = 6) receiving Priorix (GSK MMR vaccine containing MV). Seroconversion was 44%, 92% and 90% for the low, intermediate and high dose group after a single immunisation and geometric mean titres of PRNT₅₀ neutralising antibodies were 10, 48 and 46 for the low, intermediate and high vaccine priming doses with Priorix giving a control titre of 7. A homologous prime boost with an interval of 28 days yielded GM antibody titres of 73, 150 and 433 that peaked at day 56 post boost. 100% seroconversion required a vaccine boost. In general, the vaccine showed a good safety profile with no serious adverse events recorded.²⁸

A subsequent double-blind, randomised, placebo-controlled and active-controlled phase 2 trial has recently been completed. 263 participants were recruited to evaluate safety and immunogenicity after vaccination with low (5xE⁴ TCID₅₀) or high (5xE⁵ TCID₅₀) doses in 0.3 ml of solution at either, an interval of 28 (D0 and D28) or 168 (D28 and D196) days between prime and boost. Presence of neutralizing antibodies at day 56 was the primary endpoint and results showed that a low vaccine dose induced a PRNT₅₀ titer of 50.16 and 12.87 (short and long interval, respectively), while the high dose induced titers of 174.80 and 33.64 (short and long interval, respectively), with excellent safety and tolerability.²⁹

Adenoviral vectors

Adenoviruses are members of the family Adenoviridae. One of the genus belonging to this family, the Mastadenovirus have mammals and vertebrates as natural hosts and it includes the human adenoviruses. There are 51 human adenovirus serotypes which are classified in six subgroups, from A to F. Chimpanzee adenoviruses are considered part of subgroup E.³⁰

Most people have been exposed to common human adenovirus from early childhood, generating immune responses that can neutralise homologous serotypes. Neutralizing antibodies to the human serotype AdHu5, for instance, vary from 34% in the USA to 76% in Thailand and up to 89% in Nigeria.³¹ Thus, vaccine efficiency using viral vectors derived from common human serotypes can be negatively affected. Amongst approaches to circumvent pre-existing immunity to human adenovirus are the use of non-human adenoviruses.^32,33 Similarity of chimpanzee and human adenovirus has prompted the development of simian adenoviral vectors which have become widely used. The first report using a chimpanzee adenovirus as viral vectored vaccine was made by the group lead by Hildegund Ertl at the Wistar Institute in Philadelphia, USA, who demonstrated the induction of immune responses to a rabies glycoprotein expressed by the chimpanzee adenovirus serotype 68.³⁴

Recently, a new ChAdOx1 viral vector was derived from a chimpanzee adenovirus known Y25 that belongs to subgroup E adenoviruses. This adenoviral vector was shown to induce similar immunogenicity to other chimpanzee adenoviruses and low seroprevalence of pre-existing immunity in populations from the UK and Gambia.³⁵ ChAdOx1 was subsequently engineered in Oxford to express the Chikungunya structural proteins Capsid, E3, E2, 6K and E1 encoded by a consensus sequence from various CHIKV lineages. The ChAdOx1-Chik is able to induce high titres of neutralising antibodies and high frequencies of CHIKV-specific T cells in mouse pre-clinical models (manuscript in preparation).

ChAdOx1-Chik has been developed under Good Manufacturing Practices and has entered clinical trials in 2018 ClinicalTrials.gov (NCT03590392). This is a phase I, open label, dose escalation clinical trial to assess safety and immunogenicity in healthy volunteers between 18–50 years of age. The vaccine will be administered intramuscularly at three different doses: 5x10⁹, 2.5 × 10¹⁰ and 5 × 10¹⁰ that have been selected based on previous results using the ChAdOx1 viral vector as a flu vaccine.³⁶ ChAdOx1 Chik is a replication-deficient adenovirus that does not require adjuvants to induce strong immune responses and it has been administered successfully to more than 160 volunteers in various clinical trials with no serious adverse events reported. Results of the CHIK001 trial are expected in 2019.

A major consideration for vaccine approaches will be their ability to provide protection against heterologous lineages. Vaccines mentioned above have used sequences from different isolates. Table 1 shows that some are based on Asian strains, others on Waf, IOL or consensus sequences from all lineages. CHIKV has the ability to evolve into novel variants within a short period of time when entering a naive population as evidenced recently in the Americas³⁷. Nevertheless, CHIKV still keeps a high percent in amino acid identity, ranging from 95–99.9% within the structural proteins, which implies a limited diversity between CHIKV isolates. Despite the sequence and virulence diversity in animal models, vaccines based on one lineage (e.g. IOL) can provide long-lasting cross protection in heterologous CHIKV challenges in mice and macaques despite high virulence presented by some isolates in naive animals.³⁸ These conclusions have been reinforced by studies evaluating the neutralisation capacity of VLP-induced nAbs in clinical trial samples when tested against 9 CHIKV strains that represent all CHIKV genotypes.³⁹ These results indicate that CHIKV variability may not affect cross- or heterologous protection when developing vaccines based on different isolates or consensus sequences.

Another important aim for vaccine developers will consist of establishing a correlate of protection for CHIKV infection to support a swift transition towards licensure. Experience indicates that the lack of a reliable correlate of protection is a major roadblock for developing and improving vaccines such as tuberculosis⁴⁰ that rely on expensive and complex phase IIb and III trials to assess efficacy. This objective may be easier to achieve for CHIKV, as evidence indicates that the presence of IgG antibodies correlate with virus clearance.⁴¹ As CHIKV outbreaks have an unpredictable nature with rapid and unexpected movements affecting large populations to then be followed by years of relative infectious silence, efficacy trials will face a major challenge in their design to evaluate vaccine efficacy and scientists may have to find alternative and smarter strategies. Epidemiology studies will be of major importance to determine the interplay between viruses transmitted by the same Aedes mosquito species to contribute not only to the design of trials but also to vaccination strategies as CHIKV, Zika and Dengue vaccines reach the stage of licensure.

In summary, 51 years have passed since the first report of the development of a CHIKV vaccine. The unpredictable epidemiology of CHIKV with sudden massive outbreaks followed by years of relative silence, has played a major role on the development of CHIKV vaccines, which seem to gain or loose momentum depending on the emergence or disappearence of outbreaks. Initial clinical developments focused on Formalin-Inactivated Vaccines (FAV) during the late 1960s and a Live-Attenuated Vaccines (LAV) in the late 1980s. Despite promising results, efforts waned due to the unpredictability of the CHIKV epidemiology, difficulty to demonstrate protective efficacy in the field and limited funding availability.¹⁴ A CHIKV epidemic in La Réunion in 2006, maintained the interest and activities on LAV and renewed the interest in the development of new vaccine approaches, such as Virus-Like Particles (VLP) and Measles Vectored Vaccines (MVV). A more recent oubreak in the Americas starting in the Caribbean in 2013 and expanding to South, Central and North America, particularly in Mexico, contributed to the latest development based on Chimpanzee Adenoviral vectored vaccines for CHIKV that are the most recent vaccine platforms entering CHIKV clinical trials.

Funding Statement

This work was supported by the Innovate UK [972212]. https://gtr.ukri.org/projects?ref=972212

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.

Acknowledgments

Development of a ChAdOx1-Chik is independent research funded by the UK Department of Health through Innovate UK “New vaccines for global epidemics: development and manufacture” grant No. 972212 (A.R-S).