PfSPZ-CVac malaria vaccine demonstrates safety among malaria-experienced adults: A randomized, controlled phase 1 trial

BACKGROUND

Plasmodium falciparum (Pf) Sporozoite (SPZ) Chemoprophylaxis Vaccine (PfSPZ-CVac) involves concurrently administering infectious PfSPZ and malaria drug, often chloroquine (CQ), to kill liver-emerging parasites. PfSPZ-CVac (CQ) protected 100% of malaria-naïve participants against controlled human malaria infection. We investigated the hypothesis that PfSPZ-CVac (CQ) is safe and efficacious against seasonal, endemic Pf in malaria-exposed adults.

METHODS

Healthy 18-45 year olds were enrolled in a double-blind, placebo-controlled trial in Bougoula-Hameau, Mali, randomized 1:1 to 2.048 × 10⁵ PfSPZ (PfSPZ Challenge) or normal saline administered by direct venous inoculation at 0, 4, 8 weeks. Syringes were prepared by pharmacy staff using online computer-based enrolment that randomized allocations. Clinical team and participant masking was assured by identical appearance of vaccine and placebo. Participants received chloroquine 600mg before first vaccination, 10 weekly 300mg doses during vaccination, then seven daily doses of artesunate 200mg before 24-week surveillance during the rainy season. Safety outcomes were solicited adverse events (AEs) and related unsolicited AEs within 12 days of injections, and all serious AEs. Pf infection was detected by thick blood smears performed every four weeks and during febrile illness over 48 weeks. Primary vaccine efficacy (VE) endpoint was time to infection at 24 weeks. NCT02996695.

FINDINGS

62 participants were enrolled in April/May 2017. Proportions of participants experiencing at least one solicited systemic AE were similar between treatment arms: 6/31 (19.4%, 95%CI 9.2-36.3) of PfSPZ-CVac recipients versus 7/31 (22.6%, 95%CI 29.2-62.2) of controls (p value = 1.000). Two/31 (6%) in each group reported related, unsolicited AEs. One unrelated death occurred. Of 59 receiving 3 immunizations per protocol, fewer vaccinees (16/29, 55.2%) became infected than controls (22/30, 73.3%). VE was 33.6% by hazard ratio (p = 0.21, 95%CI -27·9, 65·5) and 24.8% by risk ratio (p = 0.10, 95%CI -4·8, 54·3). Antibody responses to PfCSP were poor; 28% of vaccinees sero-converted.

INTERPRETATION

PfSPZ-CVac (CQ) was well-tolerated. The tested dosing regimen failed to significantly protect against Pf infection in this very high transmission setting.

FUNDING

U.S. National Institutes of Health, Sanaria.

REGISTRATION NUMBER

ClinicalTrials.gov identifier (NCT number): NCT02996695.

Efficient infection of non-human primates with purified, cryopreserved Plasmodium knowlesi sporozoites

Background

Plasmodium falciparum (Pf) sporozoite (SPZ) vaccines are the only candidate malaria vaccines that induce > 90% vaccine efficacy (VE) against controlled human malaria infection and the only malaria vaccines to have achieved reproducible VE against malaria in adults in Africa. The goal is to increase the impact and reduce the cost of PfSPZ vaccines by optimizing vaccine potency and manufacturing, which will benefit from identification of immunological responses contributing to protection in humans. Currently, there is no authentic animal challenge model for assessing P. falciparum malaria VE. Alternatively, Plasmodium knowlesi (Pk), which infects humans and non-human primates (NHPs) in nature, can be used to experimentally infect rhesus macaques (Macaca mulatta) to assess VE.

Methods

Sanaria has, therefore, produced purified, vialed, cryopreserved PkSPZ and conducted challenge studies in several naïve NHP cohorts. In the first cohort, groups of three rhesus macaques each received doses of 5 × 10², 2.5 × 10³, 1.25 × 10⁴ and 2.5 × 10⁴ PkSPZ administered by direct venous inoculation. The infectivity of 1.5 × 10³ PkSPZ cryopreserved with an altered method and of 1.5 × 10³ PkSPZ cryopreserved for four years was tested in a second and third cohort of rhesus NHPs. The lastly, three pig-tailed macaques (Macaca nemestrina), a natural P. knowlesi host, were challenged with 2.5 × 10³ PkSPZ cryopreserved six years earlier.

Results

In the first cohort, all 12 animals developed P. knowlesi parasitaemia by thick blood smear, and the time to positivity (prepatent period) followed a non-linear 4-parameter logistic sigmoidal model with a median of 11, 10, 8, and 7 days, respectively (r² = 1). PkSPZ cryopreserved using a modified rapid-scalable method infected rhesus with a pre-patent period of 10 days, as did PkSPZ cryopreserved four years prior to infection, similar to the control group. Cryopreserved PkSPZ infected pig-tailed macaques with median time to positivity by thin smear, of 11 days.

Conclusion

This study establishes the capacity to consistently infect NHPs with purified, vialed, cryopreserved PkSPZ, providing a foundation for future studies to probe protective immunological mechanisms elicited by PfSPZ vaccines that cannot be established in humans.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-022-04261-z.

Immunological profiles associated with distinct parasitemic states in volunteers undergoing malaria challenge in Gabon

Controlled human malaria infection (CHMI) using cryopreserved non-attenuated Plasmodium falciparum sporozoites (PfSPZ) offers a unique opportunity to investigate naturally acquired immunity (NAI). By analyzing blood samples from 5 malaria-naïve European and 20 African adults with lifelong exposure to malaria, before, 5, and 11 days after direct venous inoculation (DVI) with Sanaria^R PfSPZ Challenge, we assessed the immunological patterns associated with control of microscopic and submicroscopic parasitemia. All (5/5) European individuals developed parasitemia as defined by thick blood smear (TBS), but 40% (8/20) of the African individuals controlled their parasitemia, and therefore remained thick blood smear-negative (TBS⁻ Africans). In the TBS⁻ Africans, we observed higher baseline frequencies of CD4⁺ T cells producing interferon-gamma (IFNγ) that significantly decreased 5 days after PfSPZ DVI. The TBS⁻ Africans, which represent individuals with either very strong and rapid blood-stage immunity or with immunity to liver stages, were stratified into subjects with sub-microscopic parasitemia (TBS^-PCR⁺) or those with possibly sterilizing immunity (TBS⁻PCR⁻). Higher frequencies of IFNγ⁺TNF⁺CD8⁺ γδ T cells at baseline, which later decreased within five days after PfSPZ DVI, were associated with those who remained TBS⁻PCR⁻. These findings suggest that naturally acquired immunity is characterized by different cell types that show varying strengths of malaria parasite control. While the high frequencies of antigen responsive IFNγ⁺CD4⁺ T cells in peripheral blood keep the blood-stage parasites to a sub-microscopic level, it is the IFNγ⁺TNF⁺CD8⁺ γδ T cells that are associated with either immunity to the liver-stage, or rapid elimination of blood-stage parasites.

Cellular and antibody response in GMZ2-vaccinated Gabonese volunteers in a controlled human malaria infection trial

BACKGROUND

Antibody and cellular memory responses following vaccination are important measures of immunogenicity. These immune markers were quantified in the framework of a vaccine trial investigating the malaria vaccine candidate GMZ2.

METHODS

Fifty Gabonese adults were vaccinated with two formulations (aluminum Alhydrogel and CAF01) of GMZ2 or a control vaccine (Verorab). Vaccine efficacy was assessed using controlled human malaria infection (CHMI) by direct venous inoculation of 3200 live Plasmodium falciparum sporozoites (PfSPZ Challenge). GMZ2-stimulated T and specific B-cell responses were estimated by flow cytometry before and after vaccination. Additionally, the antibody response against 212 P. falciparum antigens was estimated before CHMI by protein microarray.

RESULTS

Frequencies of pro- and anti-inflammatory CD4⁺ T cells stimulated with the vaccine antigen GMZ2 as well as B cell profiles did not change after vaccination. IL-10-producing CD4⁺ T cells and CD20⁺ IgG⁺ B cells were increased post-vaccination regardless of the intervention, thus could not be specifically attributed to any malaria vaccine regimen. In contrast, GMZ2-specific antibody response increased after the vaccination, but was not correlated to protection. Antibody responses to several P. falciparum blood and liver stage antigens (MSP1, MSP4, MSP8, PfEMP1, STARP) as well as the breadth of the malaria-specific antibody response were significantly higher in protected study participants.

CONCLUSIONS

In lifelong malaria exposed adults, the main marker of protection against CHMI is a broad antibody pattern recognizing multiple stages of the plasmodial life cycle. Despite vaccination with GMZ2 using a novel formulation, expansion of the GMZ2-stimulated T cells or the GMZ2-specific B cell response was limited, and the vaccine response could not be identified as a marker of protection against malaria. Trial registration PACTR; PACTR201503001038304; Registered 17 February 2015; https://pactr.samrc.ac.za/TrialDisplay.aspx?TrialID=1038.

Plasmodium falciparum 7G8 challenge provides conservative prediction of efficacy of PfNF54-based PfSPZ Vaccine in Africa

Controlled human malaria infection (CHMI) has supported Plasmodium falciparum (Pf) malaria vaccine development by providing preliminary estimates of vaccine efficacy (VE). Because CHMIs generally use Pf strains similar to vaccine strains, VE against antigenically heterogeneous Pf in the field has been required to establish VE. We increased the stringency of CHMI by selecting a Brazilian isolate, Pf7G8, which is genetically distant from the West African parasite (PfNF54) in our PfSPZ vaccines. Using two regimens to identically immunize US and Malian adults, VE over 24 weeks in the field was as good as or better than VE against CHMI at 24 weeks in the US. To explain this finding, here we quantify differences in the genome, proteome, and predicted CD8 T cell epitopes of PfNF54 relative to 704 Pf isolates from Africa and Pf7G8. We show that Pf7G8 is more distant from PfNF54 than any African isolates tested. We propose VE against Pf7G8 CHMI for providing pivotal data for malaria vaccine licensure for travelers to Africa, and potentially for endemic populations, because the genetic distance of Pf7G8 from the Pf vaccine strain makes it a stringent surrogate for Pf parasites in Africa.

Here the authors show that controlled human malaria infection with a Brazilian parasite highly divergent from vaccine and West African field strains can provide estimates of vaccine efficacy in Mali, and could replace field testing, streamlining vaccine development.

Clonal evolution and TCR specificity of the human TFH cell response to Plasmodium falciparum CSP

T follicular helper (T_FH) cells play a crucial role in the development of long-lived, high-quality B cell responses after infection and vaccination. However, little is known about how antigen-specific T_FH cells clonally evolve in response to complex pathogens and what guides the targeting of different epitopes. Here, we assessed the cell phenotype, clonal dynamics, and T cell receptor (TCR) specificity of human circulating T_FH (cT_FH) cells during successive malaria immunizations with radiation-attenuated Plasmodium falciparum (Pf) sporozoites. Repeated parasite exposures induced a dynamic, polyclonal cT_FH response with high frequency of cells specific to a small number of epitopes in Pf circumsporozoite protein (PfCSP), the primary sporozoite surface protein and well-defined vaccine target. Human leukocyte antigen (HLA) restrictions and differences in TCR generation probability were associated with differences in the epitope targeting frequency and indicated the potential of amino acids 311 to 333 in the Th2R/T* region as a T cell supertope. But most of vaccine-induced anti-amino acid 311 to 333 TCRs, including convergent TCRs with high sequence similarity, failed to tolerate natural polymorphisms in their target peptide sequence, thus demonstrating that the T_FH cell response was limited to the vaccine strain. These data suggest that the high parasite diversity in endemic areas will limit boosting of the vaccine-induced T_FH cell response by natural infections. Our findings may guide the further design of PfCSP-based malaria vaccines able to induce potent T helper cell responses for broad, long-lasting antibody responses.

A First for Human Vaccinology: GMP Compliant Radiation Attenuation of Plasmodium falciparum Sporozoites for Production of a Vaccine Against Malaria

Ionizing radiation (UV, X-ray and ɣ) administered at an appropriate dose to pathogenic organisms can prevent replication while preserving metabolic activity. We have established the GMP process for attenuation by ionizing radiation of the Plasmodium falciparum (Pf) sporozoites (SPZ) in Sanaria^® PfSPZ Vaccine, a protective vaccine against malaria. Mosquitoes raised and infected aseptically with Pf were transferred into infected mosquito transport containers (IMTC) and ɣ-irradiated using a ⁶⁰Co source. PfSPZ were then extracted, purified, vialed, and cryopreserved. To establish the appropriate radiation conditions, the irradiation field inside the IMTCs was mapped using radiochromic film and alanine transfer dosimeters. Dosimeters were irradiated for times calculated to provide 120-170 Gy at the minimum dose location inside the IMTC and regression analysis was used to determine the time required to achieve a lower 95% confidence interval for 150 Gy. A formula incorporating the half-life of ⁶⁰Co was then used to construct tables of irradiation times for each calendar day. From the mapping studies, formulae were derived to estimate the minimum and maximum doses of irradiation received inside the IMTC from a reference dosimeter mounted on the outside wall. For PfSPZ Vaccine manufacture a dose of 150 Gy was targeted for each irradiation event, a dose known to completely attenuate PfSPZ. The reference dosimeters were processed by the National Institute of Standards and Technology. There have been 587 irradiation events to produce PfSPZ Vaccine during 13 years which generated multiple lots released for pre-clinical studies and clinical trials. The estimated doses at the minimum dose location (mean 154.3 ± 1.77 Gy; range 150.0-159.3 Gy), and maximum dose location (mean 166.3 ± 3.65 Gy, range 155.7 to 175.3 Gy), in IMTCs were normally distributed. Overall dose uniformity was 1.078 ± 0.012. There was no siginifcant change in measured dose over 13 years. As of January 2022, 21 clinical trials of PfSPZ Vaccine have been conducted, with 1,740 volunteers aged 5 months to 61 years receiving 5,648 doses of PfSPZ Vaccine totalling >5.3 billion PfSPZ administered. There have been no breakthrough infections, confirming the consistency and robustness of the radiation attenuation process.

Cryopreserved Sporozoites with and without the Glycolipid Adjuvant 7DW8-5 Protect in Prime-and-Trap Malaria Vaccination

Repeated intravenous (IV) administration of radiation-attenuated sporozoite (RAS) vaccines induces Plasmodium-specific CD8+ liver-resident memory T (Trm) cells in mice and achieves sterile protection against challenge. Our heterologous "prime-and-trap" vaccine strategy was previously shown to simplify and improve upon RAS vaccination. Prime-and-trap vaccination combines epidermal priming by DNA-encoded circumsporozoite protein (CSP) antigen followed by a single IV dose of freshly dissected RAS (fresh-RAS) to direct and trap activated and expanding CD8+ T cells in the liver. Prime-and-trap vaccination protects mice against wild-type sporozoite (spz) challenge. Assessment of prime-and-trap vaccines in nonhuman primate (NHP) models and/or humans would be greatly enabled if fresh-RAS could be replaced by cryopreserved RAS (cryo-RAS). Here, we investigated if fresh-RAS could be replaced with cryo-RAS for prime-and-trap vaccination in BALB/cj mice. Despite a reduction in spz vaccine liver burden following cryo-RAS administration compared with fresh-RAS, cryo-RAS induced a similar level of Plasmodium yoelii (Py) CSP-specific CD8+ liver Trm cells and completely protected mice against Py spz challenge 112 days after vaccination. Additionally, when the glycolipid adjuvant 7DW8-5 was co-administered with cryo-RAS, 7DW8-5 permitted the dose of cryo-RAS to be reduced four-fold while still achieving high rates of sterile protection. In summary, cryo-RAS with and without 7DW8-5 were compatible with prime-and-trap malaria vaccination in a mouse model, which may accelerate the pathway for this vaccine strategy to move to NHPs and humans.

Multi-Dose Priming Regimens of PfSPZ Vaccine: Safety and Efficacy against Controlled Human Malaria Infection in Equatoguinean Adults

Plasmodium falciparum sporozoite (PfSPZ) Vaccine is composed of radiation-attenuated, aseptic, purified cryopreserved PfSPZ. Multiple clinical trials empirically assessing two to six doses have shown multi-dose priming (two to four doses the first week) to be optimal for protection in both 4- and 16-week regimens. In this randomized, double-blind, normal saline (NS) placebo-controlled trial, four groups (G) of 18- to 32-year-old Equatoguineans received multi-dose priming regimens with or without a delayed final dose at 4 or 16 weeks. The regimens were G1: days 1, 3, 5, 7, and 113; G2: days 1, 3, 5, and 7; G3: days 1, 3, 5, 7, and 29; and G4: days 1, 8, and 29. All doses were 9 × 105 PfSPZ. Tolerability, safety, immunogenicity, and vaccine efficacy (VE) against homologous controlled human malaria infection (CHMI) 6-7 weeks after vaccination were assessed to down-select the best regimen. All four regimens were safe and well tolerated, with no significant differences in adverse events (AEs) between vaccinees (N = 84) and NS controls (N = 20) or between regimens. Out of 19 controls, 13 developed Pf parasitemia by quantitative polymerase chain reaction (qPCR) after CHMI. Only the vaccine regimen administered on study days 1, 8, and 29 gave significant protection (7/21 vaccinees versus 13/19 controls infected, VE 51.3%, P = 0.03, Barnard's test, two-tailed). There were no significant differences in antibodies against Pf circumsporozoite protein (PfCSP), a major SPZ antigen, between protected and nonprotected vaccinees or controls pre-CHMI. The six controls not developing Pf parasitemia had significantly higher antibodies to blood stage antigens Pf exported protein 1 (PfEXP1) and Pf merozoite surface protein 1 (PfMSP1) than the controls who developed parasitemia, suggesting naturally acquired immunity against Pf limited infections in controls. This study identified a safe, protective, 4-week, multi-dose prime vaccination regimen for assessment in future trials of PfSPZ Vaccine.

Diagnostic performance and comparison of ultrasensitive and conventional rapid diagnostic test, thick blood smear and quantitative PCR for detection of low-density Plasmodium falciparum infections during a controlled human malaria infection study in Equatorial Guinea

Background

Progress towards malaria elimination has stagnated, partly because infections persisting at low parasite densities comprise a large reservoir contributing to ongoing malaria transmission and are difficult to detect. This study compared the performance of an ultrasensitive rapid diagnostic test (uRDT) designed to detect low density infections to a conventional RDT (cRDT), expert microscopy using Giemsa-stained thick blood smears (TBS), and quantitative polymerase chain reaction (qPCR) during a controlled human malaria infection (CHMI) study conducted in malaria exposed adults (NCT03590340).

Methods

Blood samples were collected from healthy Equatoguineans aged 18–35 years beginning on day 8 after CHMI with 3.2 × 10³ cryopreserved, infectious Plasmodium falciparum sporozoites (PfSPZ Challenge, strain NF54) administered by direct venous inoculation. qPCR (18s ribosomal DNA), uRDT (Alere™ Malaria Ag P.f.), cRDT [Carestart Malaria Pf/PAN (PfHRP2/pLDH)], and TBS were performed daily until the volunteer became TBS positive and treatment was administered. qPCR was the reference for the presence of Plasmodium falciparum parasites.

Results

279 samples were collected from 24 participants; 123 were positive by qPCR. TBS detected 24/123 (19.5% sensitivity [95% CI 13.1–27.8%]), uRDT 21/123 (17.1% sensitivity [95% CI 11.1–25.1%]), cRDT 10/123 (8.1% sensitivity [95% CI 4.2–14.8%]); all were 100% specific and did not detect any positive samples not detected by qPCR. TBS and uRDT were more sensitive than cRDT (TBS vs. cRDT p = 0.015; uRDT vs. cRDT p = 0.053), detecting parasitaemias as low as 3.7 parasites/µL (p/µL) (TBS and uRDT) compared to 5.6 p/µL (cRDT) based on TBS density measurements. TBS, uRDT and cRDT did not detect any of the 70/123 samples positive by qPCR below 5.86 p/µL, the qPCR density corresponding to 3.7 p/µL by TBS. The median prepatent periods in days (ranges) were 14.5 (10–20), 18.0 (15–28), 18.0 (15–20) and 18.0 (16–24) for qPCR, TBS, uRDT and cRDT, respectively; qPCR detected parasitaemia significantly earlier (3.5 days) than the other tests.

Conclusions

TBS and uRDT had similar sensitivities, both were more sensitive than cRDT, and neither matched qPCR for detecting low density parasitaemia. uRDT could be considered an alternative to TBS in selected applications, such as CHMI or field diagnosis, where qualitative, dichotomous results for malaria infection might be sufficient.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-022-04103-y.

Safety and efficacy of a three-dose regimen of Plasmodium falciparum sporozoite vaccine in adults during an intense malaria transmission season in Mali: a randomised, controlled phase 1 trial

BACKGROUND

WHO recently approved a partially effective vaccine that reduces clinical malaria in children, but increased vaccine activity is required to pursue malaria elimination. A phase 1 clinical trial was done in Mali, west Africa, to assess the safety, immunogenicity, and protective efficacy of a three-dose regimen of Plasmodium falciparum sporozoite (PfSPZ) Vaccine (a metabolically active, non-replicating, whole malaria sporozoite vaccine) against homologous controlled human malaria infection (CHMI) and natural P falciparum infection.

METHODS

We recruited healthy non-pregnant adults aged 18-50 years in Donéguébougou, Mali, and surrounding villages (Banambani, Toubana, Torodo, Sirababougou, Zorokoro) for an open-label, dose-escalation pilot study and, thereafter, a randomised, double-blind, placebo-controlled main trial. Pilot study participants were enrolled on an as-available basis to one group of CHMI infectivity controls and three staggered vaccine groups receiving: one dose of 4·5 × 105, one dose of 9 × 105, or three doses of 1·8 × 106 PfSPZ via direct venous inoculation at approximately 8 week intervals, followed by homologous CHMI 5 weeks later with infectious PfSPZ by direct venous inoculation (PfSPZ Challenge). Main cohort participants were stratified by village and randomly assigned (1:1) to receive three doses of 1·8 × 106 PfSPZ or normal saline at 1, 13, and 19 week intervals using permuted block design by the study statistician. The primary outcome was safety and tolerability of at least one vaccine dose; the secondary outcome was vaccine efficacy against homologous PfSPZ CHMI (pilot study) or against naturally transmitted P falciparum infection (main study) measured by thick blood smear. Combined artesunate and amodiaquine was administered to eliminate pre-existing parasitaemia. Outcomes were analysed by modified intention to treat (mITT; including all participants who received at least one dose of investigational product; safety and vaccine efficacy) and per protocol (vaccine efficacy). This trial is registered with ClinicalTrials.gov, number NCT02627456.

FINDINGS

Between Dec 20, 2015, and April 30, 2016, we enrolled 56 participants into the pilot study (five received the 4·5 × 105 dose, five received 9 × 105, 30 received 1·8 × 106, 15 were CHMI controls, and one withdrew before vaccination) and 120 participants into the main study cohort with 60 participants assigned PfSPZ Vaccine and 60 placebo in the main study. Adverse events and laboratory abnormalities post-vaccination in all dosing groups were few, mainly mild, and did not differ significantly between vaccine groups (all p>0·05). Unexpected severe transaminitis occured in four participants: one participant in pilot phase that received 1·8 × 106 PfSPZ Vaccine, one participant in main phase that received 1·8 × 106 PfSPZ Vaccine, and two participants in the main phase placebo group. During PfSPZ CHMI, approximately 5 weeks after the third dose of 1·8 × 106 PfSPZ, none of 29 vaccinees and one of 15 controls became positive on thick blood smear; subsequent post-hoc PCR analysis for submicroscopic blood stage infections detected P falciparum parasites in none of the 29 vaccine recipients and eight of 15 controls during CHMI. In the main trial, 32 (58%) of 55 vaccine recipients and 42 (78%) of 54 controls became positive on thick blood smear during 24-week surveillance after vaccination. Vaccine efficacy (1-hazard ratio) was 0·51 per protocol (95% CI 0·20-0·70; log-rank p=0·0042) and 0·39 by mITT (0·04-0·62; p=0·033); vaccine efficacy (1-risk ratio) was 0·24 per-protocol (0·02-0·41; p=0·031) and 0·22 mITT (0·01-0·39; p=0·041).

INTERPRETATION

A three-dose regimen of PfSPZ Vaccine was safe, well tolerated, and conferred 51% vaccine efficacy against intense natural P falciparum transmission, similar to 52% vaccine efficacy reported for a five-dose regimen in a previous trial.

FUNDING

US National Institute of Allergy and Infectious Diseases, National Institutes of Health, Sanaria.

TRANSLATION

For the French translation of the abstract see Supplementary Materials section.

Controlled human malaria infection (CHMI) outcomes in Kenyan adults is associated with prior history of malaria exposure and anti-schizont antibody response

Background

Individuals living in endemic areas acquire immunity to malaria following repeated parasite exposure. We sought to assess the controlled human malaria infection (CHMI) model as a means of studying naturally acquired immunity in Kenyan adults with varying malaria exposure.

Methods

We analysed data from 142 Kenyan adults from three locations representing distinct areas of malaria endemicity (Ahero, Kilifi North and Kilifi South) enrolled in a CHMI study with Plasmodium falciparum sporozoites NF54 strain (Sanaria® PfSPZ Challenge). To identify the in vivo outcomes that most closely reflected naturally acquired immunity, parameters based on qPCR measurements were compared with anti-schizont antibody levels and residence as proxy markers of naturally acquired immunity.

Results

Time to endpoint correlated more closely with anti-schizont antibodies and location of residence than other parasite parameters such as growth rate or mean parasite density. Compared to observational field-based studies in children where 0.8% of the variability in malaria outcome was observed to be explained by anti-schizont antibodies, in the CHMI model the dichotomized anti-schizont antibodies explained 17% of the variability.

Conclusions

The CHMI model is highly effective in studying markers of naturally acquired immunity to malaria.

Trial registration Clinicaltrials.gov number NCT02739763. Registered 15 April 2016

Cryopreservation of Anopheles stephensi embryos

The ability to cryopreserve mosquitoes would revolutionize work on these vectors of major human infectious diseases by conserving stocks, new isolates, lab-bred strains, and transgenic lines that currently require continuous life cycle maintenance. Efforts over several decades to develop a method for cryopreservation have, until now, been fruitless: we describe here a method for the cryopreservation of Anopheles stephensi embryos yielding hatch rates of ~ 25%, stable for > 5 years. Hatched larvae developed into fertile, fecund adults and blood-fed females, produced fully viable second generation eggs, that could be infected with Plasmodium falciparum at high intensities. The key components of the cryopreservation method are: embryos at 15-30 min post oviposition, two incubation steps in 100% deuterated methanol at - 7 °C and - 14.5 °C, and rapid cooling. Eggs are recovered by rapid warming with concomitant dilution of cryoprotectant. Eggs of genetically modified A. stephensi and of A. gambiae were also successfully cryopreserved. This enabling methodology will allow long-term conservation of mosquitoes as well as acceleration of genetic studies and facilitation of mass storage of anopheline mosquitoes for release programs.

OUP accepted manuscript

Loa loa microfilariae were found on thick blood smears (TBSs) from 8 of 300 (2.7%) residents of Bioko Island, Equatorial Guinea, during a Plasmodium falciparum sporozoite malaria vaccine clinical trial. Only one subject was found to have microfilaraemia on his first exam; parasites were not discovered in the other seven until subsequent TBSs were performed, at times many weeks into the study. All infected individuals were asymptomatic, and were offered treatment with diethylcarbamazine, per national guidelines. L. loa microfilaraemia complicated the enrolment or continued participation of these eight trial subjects, and only one was able to complete all study procedures. If ruling out loiasis is deemed to be important during clinical trials, tests that are more sensitive than TBSs should be performed.

An efficient single-cell based method for linking human T cell phenotype to T cell receptor sequence and specificity

Single-cell antigen-receptor gene amplification and sequencing platforms have been used to characterize T cell receptor (TCR) repertoires but typically fail to generate paired full-length gene products for direct expression cloning and do not enable linking this data to cell phenotype information. To overcome these limitations, we established a high-throughput platform for the quantitative and qualitative analysis of human TCR repertoires that provides insights into the clonal and functional composition of human CD4⁺ and CD8⁺ αβ T cells at the molecular and cellular level. The strategy is a powerful tool to qualitatively assess differences between antigen receptors of phenotypically defined αβ T cell subsets, e.g. in immune responses to cancer, vaccination, or infection, and in autoimmune diseases.

Identification of the Glycan Binding Profile of Human and Rodent Plasmodium Sporozoites

The transmission of Plasmodium spp. sporozoites to the mammalian host is the first step in the initiation of the mosquito-borne disease known as malaria. The exact route of transmission from the bloodstream to the liver is still not clearly elucidated, and identification of the host glycan structures bound by the sporozoites may inform as to which host cells are involved. Here, we provide a comprehensive analysis of the glycan structures that sporozoites from the human pathogen, P. falciparum, and the rodent pathogen, P. yoelii, recognize and bind. Glycan array analysis was used to profile the glycans bound by the sporozoites, and the binding affinities of these sporozoite-glycan interactions were then determined by surface plasmon resonance. Data showed that the different Plasmodium spp. bind different classes of glycans. P. falciparum was observed to bind to glycans with terminal N-acetylgalactosamine (GalNAc) or Galactose (Gal) linked to a GalNAc, and the highest-affinity observed was with the GalNAc monosaccharide (12.5 nM). P. yoelii bound glycosaminoglycans, mannosyl glycans, Gal linked to N-acetylglucosamine structures, and the αGal epitope. The highest-affinity interaction for P. yoelii was with the αGal epitope (31.4 nM). This is the first study to identify the key host glycan structures recognized by human and rodent Plasmodium spp. sporozoites. An understanding of how Plasmodium sporozoites interact with the specific glycan structures identified here may provide further insight into this infectious disease that could help direct the design of an effective therapeutic.

Incidence of Plasmodium falciparum malaria infection in 6-month to 45-year-olds on selected areas of Bioko Island, Equatorial Guinea

Background

Extensive malaria control measures have been implemented on Bioko Island, Equatorial Guinea over the past 16 years, reducing parasite prevalence and malaria-related morbidity and mortality, but without achieving elimination. Malaria vaccines offer hope for reducing the burden to zero. Three phase 1/2 studies have been conducted successfully on Bioko Island to evaluate the safety and efficacy of whole Plasmodium falciparum (Pf) sporozoite (SPZ) malaria vaccines. A large, pivotal trial of the safety and efficacy of the radiation-attenuated Sanaria^® PfSPZ Vaccine against P. falciparum is planned for 2022. This study assessed the incidence of malaria at the phase 3 study site and characterized the influence of socio-demographic factors on the burden of malaria to guide trial design.

Methods

A cohort of 240 randomly selected individuals aged 6 months to 45 years from selected areas of North Bioko Province, Bioko Island, was followed for 24 weeks after clearance of parasitaemia. Assessment of clinical presentation consistent with malaria and thick blood smears were performed every 2 weeks. Incidence of first and multiple malaria infections per person-time of follow-up was estimated, compared between age groups, and examined for associated socio-demographic risk factors.

Results

There were 58 malaria infection episodes observed during the follow up period, including 47 first and 11 repeat infections. The incidence of malaria was 0.25 [95% CI (0.19, 0.32)] and of first malaria was 0.23 [95% CI (0.17, 0.30)] per person per 24 weeks (0.22 in 6–59-month-olds, 0.26 in 5–17-year-olds, 0.20 in 18–45-year-olds). Incidence of first malaria with symptoms was 0.13 [95% CI (0.09, 0.19)] per person per 24 weeks (0.16 in 6–59-month-olds, 0.10 in 5–17-year-olds, 0.11 in 18–45-year-olds). Multivariate assessment showed that study area, gender, malaria positivity at screening, and household socioeconomic status independently predicted the observed incidence of malaria.

Conclusion

Despite intensive malaria control efforts on Bioko Island, local transmission remains and is spread evenly throughout age groups. These incidence rates indicate moderate malaria transmission which may be sufficient to support future larger trials of PfSPZ Vaccine. The long-term goal is to conduct mass vaccination programmes to halt transmission and eliminate P. falciparum malaria.

Early whole blood transcriptional responses to radiation-attenuated Plasmodium falciparum sporozoite vaccination in malaria naïve and malaria pre-exposed adult volunteers

Background

Vaccination with radiation-attenuated Plasmodium falciparum sporozoites is known to induce protective immunity. However, the mechanisms underlying this protection remain unclear. In this work, two recent radiation-attenuated sporozoite vaccination studies were used to identify potential transcriptional correlates of vaccination-induced protection.

Methods

Longitudinal whole blood RNAseq transcriptome responses to immunization with radiation-attenuated P. falciparum sporozoites were analysed and compared across malaria-naïve adult participants (IMRAS) and malaria-experienced adult participants (BSPZV1). Parasite dose and method of delivery differed between trials, and immunization regimens were designed to achieve incomplete protective efficacy. Observed protective efficacy was 55% in IMRAS and 20% in BSPZV1. Study vaccine dosings were chosen to elicit both protected and non-protected subjects, so that protection-associated responses could be identified.

Results

Analysis of comparable time points up to 1 week after the first vaccination revealed a shared cross-study transcriptional response programme, despite large differences in number and magnitude of differentially expressed genes between trials. A time-dependent regulatory programme of coherent blood transcriptional modular responses was observed, involving induction of inflammatory responses 1–3 days post-vaccination, with cell cycle responses apparent by day 7 in protected individuals from both trials. Additionally, strongly increased induction of inflammation and interferon-associated responses was seen in non-protected IMRAS participants. All individuals, except for non-protected BSPZV1 participants, showed robust upregulation of cell-cycle associated transcriptional responses post vaccination.

Conclusions

In summary, despite stark differences between the two studies, including route of vaccination and status of malaria exposure, responses were identified that were associated with protection after PfRAS vaccination. These comprised a moderate early interferon response peaking 2 days post vaccination, followed by a later proliferative cell cycle response steadily increasing over the first 7 days post vaccination. Non-protection is associated with deviations from this model, observed in this study with over-induction of early interferon responses in IMRAS and failure to mount a cell cycle response in BSPZV1.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-021-03839-3.

Two chemoattenuated PfSPZ malaria vaccines induce sterile hepatic immunity

The global decline in malaria has stalled1, emphasizing the need for vaccines that induce durable sterilizing immunity. Here we optimized regimens for chemoprophylaxis vaccination (CVac), for which aseptic, purified, cryopreserved, infectious Plasmodium falciparum sporozoites (PfSPZ) were inoculated under prophylactic cover with pyrimethamine (PYR) (Sanaria PfSPZ-CVac(PYR)) or chloroquine (CQ) (PfSPZ-CVac(CQ))-which kill liver-stage and blood-stage parasites, respectively-and we assessed vaccine efficacy against homologous (that is, the same strain as the vaccine) and heterologous (a different strain) controlled human malaria infection (CHMI) three months after immunization ( https://clinicaltrials.gov/ , NCT02511054 and NCT03083847). We report that a fourfold increase in the dose of PfSPZ-CVac(PYR) from 5.12 × 104 to 2 × 105 PfSPZs transformed a minimal vaccine efficacy (low dose, two out of nine (22.2%) participants protected against homologous CHMI), to a high-level vaccine efficacy with seven out of eight (87.5%) individuals protected against homologous and seven out of nine (77.8%) protected against heterologous CHMI. Increased protection was associated with Vδ2 γδ T cell and antibody responses. At the higher dose, PfSPZ-CVac(CQ) protected six out of six (100%) participants against heterologous CHMI three months after immunization. All homologous (four out of four) and heterologous (eight out of eight) infectivity control participants showed parasitaemia. PfSPZ-CVac(CQ) and PfSPZ-CVac(PYR) induced a durable, sterile vaccine efficacy against a heterologous South American strain of P. falciparum, which has a genome and predicted CD8 T cell immunome that differs more strongly from the African vaccine strain than other analysed African P. falciparum strains.

Transient knockdown of Anopheles stephensi LRIM1 using RNAi increases Plasmodium falciparum sporozoite salivary gland infections

BACKGROUND

Plasmodium falciparum (Pf) sporozoites (PfSPZ) can be administered as a highly protective vaccine conferring the highest protection seen to date. Sanaria® PfSPZ vaccines are produced using aseptically reared Anopheles stephensi mosquitoes. The bionomics of sporogonic development of P. falciparum in A. stephensi to fully mature salivary gland PfSPZ is thought to be modulated by several components of the mosquito innate immune system. In order to increase salivary gland PfSPZ infections in A. stephensi and thereby increase vaccine production efficiency, a gene knock down approach was used to investigate the activity of the immune deficiency (IMD) signaling pathway downstream effector leucine-rich repeat immune molecule 1 (LRIM1), an antagonist to Plasmodium development.

METHODS

Expression of LRIM1 in A. stephensi was reduced following injection of double stranded (ds) RNA into mosquitoes. By combining the Gal4/UAS bipartite system with in vivo expression of short hairpin (sh) RNA coding for LRIM1 reduced expression of LRIM1 was targeted in the midgut, fat body, and salivary glands. RT-qPCR was used to demonstrate fold-changes in gene expression in three transgenic crosses and the effects on P. falciparum infections determined in mosquitoes showing the greatest reduction in LRIM1 expression.

RESULTS

LRIM1 expression could be reduced, but not completely silenced, by expression of LRIM1 dsRNA. Infections of P. falciparum oocysts and PfSPZ were consistently and significantly higher in transgenic mosquitoes than wild type controls, with increases in PfSPZ ranging from 2.5- to tenfold.

CONCLUSIONS

Plasmodium falciparum infections in A. stephensi can be increased following reduced expression of LRIM1. These data provide the springboard for more precise knockout of LRIM1 for the eventual incorporation of immune-compromised A. stephensi into manufacturing of Sanaria's PfSPZ products.

A double-blind, placebo-controlled phase 1/2a trial of the genetically attenuated malaria vaccine PfSPZ-GA1

Immunization with attenuated Plasmodium sporozoites can induce protection against malaria infection, as shown by Plasmodium falciparum (Pf) sporozoites attenuated by radiation in multiple clinical trials. As alternative attenuation strategy with a more homogeneous population of Pf sporozoites (PfSPZ), genetically engineered Plasmodium berghei sporozoites (SPZ) lacking the genes b9 and slarp induced sterile protection against malaria in mice. Consequently, PfSPZ-GA1 Vaccine, a Pf identical double knockout (Pf∆b9∆slarp), was generated as a genetically attenuated malaria parasite vaccine and tested for safety, immunogenicity, and preliminary efficacy in malaria-naïve Dutch volunteers. Dose-escalation immunizations up to 9.0 × 10⁵ PfSPZ of PfSPZ-GA1 Vaccine were well tolerated without breakthrough blood-stage infection. Subsequently, groups of volunteers were immunized three times by direct venous inoculation with cryopreserved PfSPZ-GA1 Vaccine (9.0 × 10⁵ or 4.5 × 10⁵ PfSPZ, N = 13 each), PfSPZ Vaccine (radiation-attenuated PfSPZ, 4.5 × 10⁵ PfSPZ, N = 13), or normal saline placebo at 8-week intervals, followed by exposure to mosquito bite controlled human malaria infection (CHMI). After CHMI, 3 of 25 volunteers from both PfSPZ-GA1 groups were sterilely protected, and the remaining 17 of 22 showed a patency ≥9 days (median patency in controls, 7 days; range, 7 to 9). All volunteers in the PfSPZ Vaccine control group developed parasitemia (median patency, 9 days; range, 7 to 12). Immunized groups exhibited a significant, dose-related increase in anti-Pf circumsporozoite protein (CSP) antibodies and Pf-specific interferon-γ (IFN-γ)-producing T cells. Although no definite conclusion can be drawn on the potential strength of protective efficacy of PfSPZ-GA1 Vaccine, the favorable safety profile and induced immune responses by PfSPZ-GA1 Vaccine warrant further clinical evaluation.

Exploratory analysis of the effect of helminth infection on the immunogenicity and efficacy of the asexual blood-stage malaria vaccine candidate GMZ2

Background

Helminths can modulate the host immune response to Plasmodium falciparum and can therefore affect the risk of clinical malaria. We assessed here the effect of helminth infections on both the immunogenicity and efficacy of the GMZ2 malaria vaccine candidate, a recombinant protein consisting of conserved domains of GLURP and MSP3, two asexual blood-stage antigens of P. falciparum. Controlled human malaria infection (CHMI) was used to assess the efficacy of the vaccine.

Methodology

In a randomized, double-blind Phase I clinical trial, fifty, healthy, lifelong malaria-exposed adult volunteers received three doses of GMZ2 adjuvanted with either Cationic Adjuvant Formulation (CAF) 01 or Alhydrogel, or a control vaccine (Rabies) on days (D) 0, D28 and D56, followed by direct venous inoculation (DVI) of 3,200 P. falciparum sporozoites (PfSPZ Challenge) approximately 13 weeks after last vaccination to assess vaccine efficacy. Participants were followed-up on a daily basis with clinical examinations and thick blood smears to monitor P. falciparum parasitemia for 35 days. Malaria was defined as the presence of P. falciparum parasites in the blood associated with at least one symptom that can be associated to malaria over 35 days following DVI of PfSPZ Challenge. Soil-transmitted helminth (STH) infection was assessed by microscopy and by polymerase chain reaction (PCR) on stool, and Schistosoma infection was assessed by microscopy on urine. Participants were considered as infected if positive for any helminth either by PCR and/or microscopy at D0 and/or at D84 (Helm+) and were classified as mono-infection or co-infection. Total vaccine-specific IgG concentrations assessed on D84 were analysed as immunogenicity outcome.

Main findings

The helminth in mono-infection, particularly Schistosoma haematobium and STH were significantly associated with earlier malaria episodes following CHMI, while no association was found in case of coinfection. In further analyses, the anti-GMZ2 IgG concentration on D84 was significantly higher in the S. haematobium-infected and significantly lower in the Strongyloides stercoralis-infected groups, compared to helminth-negative volunteers. Interesting, in the absence of helminth infection, a high anti-GMZ2 IgG concentration on D84 was significantly associated with protection against malaria.

Conclusions

Our results suggest that helminth infection may reduce naturally acquired and vaccine-induced protection against malaria. Vaccine-specific antibody concentrations on D84 may be associated with protection in participants with no helminth infection. These results suggest that helminth infection affect malaria vaccine immunogenicity and efficacy in helminth endemic countries.

Helminths, mainly because of their immune regulatory effects, are able to impact the response induced by vaccines. In the context of clinical trial designs that measure accrual of natural infections during follow up or outcome of controlled human malaria infection (CHMI), their effect on vaccine efficacy can be measured. Indeed, most of such clinical trials on malaria vaccine candidates conducted in Africa, especially where the prevalence of helminths is high, have shown a certain limit in their efficacy and immunogenicity, as compared to results observed in European and U.S volunteers. The present analysis assessed the effect of helminths on GMZ2, a malaria vaccine candidate. We found a high level of anti-GMZ2 antibodies among volunteers not infected with helminths and protected against CHMI, indicating efficacy of the candidate vaccine in this population. We found a species-dependent effect of helminths on the level of post-immunization GMZ2-specific IgG concentration, and an association of helminths with an early onset of malaria in CHMI. Our findings reveal that helminths are associated with immunogenicity and may decrease the protective effect of antibodies induced by vaccination. Helminth infection status shall be determined when measuring the immunogenicity and efficacy of malaria vaccine candidates in helminth endemic countries.

PfSPZ-CVac efficacy against malaria increases from 0% to 75% when administered in the absence of erythrocyte stage parasitemia: A randomized, placebo-controlled trial with controlled human malaria infection

PfSPZ-CVac combines 'PfSPZ Challenge', which consists of infectious Plasmodium falciparum sporozoites (PfSPZ), with concurrent antimalarial chemoprophylaxis. In a previously-published PfSPZ-CVac study, three doses of 5.12x104 PfSPZ-CVac given 28 days apart had 100% vaccine efficacy (VE) against controlled human malaria infection (CHMI) 10 weeks after the last immunization, while the same dose given as three injections five days apart had 63% VE. Here, we conducted a dose escalation trial of similarly condensed schedules. Of the groups proceeding to CHMI, the first study group received three direct venous inoculations (DVIs) of a dose of 5.12x104 PfSPZ-CVac seven days apart and the next full dose group received three DVIs of a higher dose of 1.024x105 PfSPZ-CVac five days apart. CHMI (3.2x103 PfSPZ Challenge) was performed by DVI 10 weeks after the last vaccination. In both CHMI groups, transient parasitemia occurred starting seven days after each vaccination. For the seven-day interval group, the second and third vaccinations were therefore administered coincident with parasitemia from the prior vaccination. Parasitemia was associated with systemic symptoms which were severe in 25% of subjects. VE in the seven-day group was 0% (7/7 infected) and in the higher-dose, five-day group was 75% (2/8 infected). Thus, the same dose of PfSPZ-CVac previously associated with 63% VE when given on a five-day schedule in the prior study had zero VE here when given on a seven-day schedule, while a double dose given on a five-day schedule here achieved 75% VE. The relative contributions of the five-day schedule and/or the higher dose to improved VE warrant further investigation. It is notable that administration of PfSPZ-CVac on a schedule where vaccine administration coincided with blood-stage parasitemia was associated with an absence of sterile protective immunity. Clinical trials registration: NCT02773979.

Increase of Dose Associated With Decrease in Protection Against Controlled Human Malaria Infection by PfSPZ Vaccine in Tanzanian Adults

BACKGROUND

A vaccine would be an ideal tool for reducing malaria's impact. PfSPZ Vaccine (radiation attenuated, aseptic, purified, cryopreserved Plasmodium falciparum [Pf] sporozoites [SPZ]) has been well tolerated and safe in >1526 malaria-naive and experienced 6-month to 65-year-olds in the United States, Europe, and Africa. When vaccine efficacy (VE) of 5 doses of 2.7 × 105 PfSPZ of PfSPZ Vaccine was assessed in adults against controlled human malaria infection (CHMI) in the United States and Tanzania and intense field transmission of heterogeneous Pf in Mali, Tanzanians had the lowest VE (20%).

METHODS

To increase VE in Tanzania, we increased PfSPZ/dose (9 × 105 or 1.8 × 106) and decreased numbers of doses to 3 at 8-week intervals in a double blind, placebo-controlled trial.

RESULTS

All 22 CHMIs in controls resulted in parasitemia by quantitative polymerase chain reaction. For the 9 × 105 PfSPZ group, VE was 100% (5/5) at 3 or 11 weeks (P < .000l, Barnard test, 2-tailed). For 1.8 × 106 PfSPZ, VE was 33% (2/6) at 7.5 weeks (P = .028). VE of dosage groups (100% vs 33%) was significantly different (P = .022). Volunteers underwent repeat CHMI at 37-40 weeks after last dose. 6/6 and 5/6 volunteers developed parasitemia, but time to first parasitemia was significantly longer than controls in the 9 × 105 PfSPZ group (10.89 vs 7.80 days) (P = .039), indicating a significant reduction in parasites in the liver. Antibody and T-cell responses were higher in the 1.8 × 106 PfSPZ group.

CONCLUSIONS

In Tanzania, increasing the dose from 2.7 × 105 to 9 × 105 PfSPZ increased VE from 20% to 100%, but increasing to 1.8 × 106 PfSPZ significantly reduced VE.

CLINICAL TRIALS REGISTRATION

NCT02613520.

Safety, Tolerability, and Immunogenicity of Plasmodium falciparum Sporozoite Vaccine Administered by Direct Venous Inoculation to Infants and Young Children: Findings From an Age De-escalation, Dose-Escalation, Double-blind, Randomized Controlled Study in Western Kenya

BACKGROUND

The whole Plasmodium falciparum sporozoite (PfSPZ) vaccine is being evaluated for malaria prevention. The vaccine is administered intravenously for maximal efficacy. Direct venous inoculation (DVI) with PfSPZ vaccine has been safe, tolerable, and feasible in adults, but safety data for children and infants are limited.

METHODS

We conducted an age de-escalation, dose-escalation randomized controlled trial in Siaya County, western Kenya. Children and infants (aged 5-9 years, 13-59 months, and 5-12 months) were enrolled into 13 age-dose cohorts of 12 participants and randomized 2:1 to vaccine or normal saline placebo in escalating doses: 1.35 × 105, 2.7 × 105, 4.5 × 105, 9.0 × 105, and 1.8 × 106 PfSPZ, with the 2 highest doses given twice, 8 weeks apart. Solicited adverse events (AEs) were monitored for 8 days after vaccination, unsolicited AEs for 29 days, and serious AEs throughout the study. Blood taken prevaccination and 1 week postvaccination was tested for immunoglobulin G antibodies to P. falciparum circumsporozoite protein (PfCSP) using enzyme-linked immunosorbent assay.

RESULTS

Rates of AEs were similar in vaccinees and controls for solicited (35.7% vs 41.5%) and unsolicited (83.9% vs 92.5%) AEs, respectively. No related grade 3 AEs, serious AEs, or grade 3 laboratory abnormalities occurred. Most (79.0%) vaccinations were administered by a single DVI. Among those in the 9.0 × 105 and 1.8 × 106 PfSPZ groups, 36 of 45 (80.0%) vaccinees and 4 of 21 (19.0%) placebo controls developed antibodies to PfCSP (P < .001).

CONCLUSIONS

PfSPZ vaccine in doses as high as 1.8 × 106 can be administered to infants and children by DVI, and was safe, well tolerated, and immunogenic.

CLINICAL TRIALS REGISTRATION

NCT02687373.