Preliminary nonclinical safety and immunogenicity of an rVSV-ΔG-SARS-CoV-2-S vaccine in mice, hamsters, rabbits and pigs

Abstract

rVSV-ΔG-SARS-CoV-2-S is a clinical stage (Phase 2) replication competent recombinant vaccine against SARS-CoV-2. To evaluate the safety profile of the vaccine, a series of non-clinical safety, immunogenicity and efficacy studies were conducted in four animal species, using multiple doses (up to 10⁸ Plaque Forming Units/animal) and dosing regimens. There were no treatment-related mortalities or any noticeable clinical signs in any of the studies. Compared to unvaccinated controls, hematology and biochemistry parameters were unremarkable and no adverse histopathological findings. There was no detectable viral shedding in urine, nor viral RNA detected in whole blood or serum samples seven days post vaccination. The rVSV-ΔG-SARS-CoV-2-S vaccination gave rise to neutralizing antibodies, cellular immune responses, and increased lymphocytic cellularity in the spleen germinal centers and regional lymph nodes. No evidence for neurovirulence was found in C57BL/6 immune competent mice or in highly sensitive type I interferon knock-out mice. Vaccine virus replication and distribution in K18-human Angiotensin-converting enzyme 2-transgenic mice showed a gradual clearance from the vaccination site with no vaccine virus recovered from the lungs. The nonclinical data suggest that the rVSV-ΔG-SARS-CoV-2-S vaccine is safe and immunogenic. These results supported the initiation of clinical trials, currently in Phase 2.

Keywords: COVID-19; Neutralizing antibodies; Nonclinical; Safety; Vaccine.

Fig. 1

Body weight, temperature and serum neutralizing antibody titers in rabbits after prime/boost vaccination. a Body weight change percentage compared between groups (n = 4 /group). b Group mean body temperature (°C) following prime and boost vaccination of rabbits. Dashed lines indicate normal body temperature range. c Vaccinated rabbit sera neutralization assay results [mean group NT50, using plaque reduction neutralization test (PRNT)]. Sera were collected on day 14 (14 days post first vaccination; “prime”) and on day 31 (17 days post the second vaccination; “boost”). Dotted line represents the limit of detection (LOD). Error bars show median interquartile range (IQR). Means and SD are indicated below the graph

Fig. 2

Rabbit histopathology following prime/boost vaccination. Histopathological evaluation performed 3 weeks post second-vaccination session. a–b Injection-Site Analysis (high dose, 10⁸ PFU/animal)- arrows indicate changes at the injection sites consisting of focal minimal mixed inflammatory cell infiltration (a) (× 100) and focal minimal muscle fiber necrosis associated with mixed inflammatory cell infiltration (b) (× 400) which are not considered as adverse treatment effects. c Spleen of control animal (× 100) and d high dose (10⁸ PFU) animal (× 100): arrows indicate minimal to mild germinal centers increased lymphocytic cellularity (i.e., follicular hyperplasia), noted following vaccination and considered to reflect a secondary change due to antigenic stimulation. No evidence of germinal centers was observed in a control animal (c). e Iliac Lymph Node (regional lymph node to the injection site) of control animal (× 40) and (f) of the high dose (10⁸ PFU) animal (× 40): arrows indicate mild to moderate germinal centers increased lymphocytic cellularity (i.e., follicular hyperplasia), noted following vaccination and considered to reflect a secondary change due to antigenic stimulation. No evidence of germinal centers was observed in a control animal (e)

Fig. 3

Body weight, temperature and serum neutralizing antibody titers in pigs after prime/boost vaccination. a Individual body weight change (percentage). b Individual body temperature (°C) following prime and boost vaccination of pigs. Dashed lines indicate normal body temperature range. c Individual pig sera neutralization assay results (mean group NT50, using plaque reduction neutralization test (PRNT). Sera were collected on day 14 (14 days post first vaccination; “prime”) and on day 31 (“boost”). Error bars show median interquartile range (IQR)

Fig. 4

Cytokine secretions in pigs after prime/boost vaccination. a IFNγ secretion by Elispot assay: Antigen induced IFNγ producing T cells (spots/10⁶ PBMCs). b Cytokine secretion three weeks post boost dose in pigs: Cytokine measurements were in duplicates for no antigen controls and naïve animals, and were in quadruplates for pigs vaccinated with rVSV-SARS-CoV-2-S. Positive controls (not shown) were measured for each cytokine in duplicate

Fig. 5

Vaccine viral loads at the injection site in K18-hACE2 transgenic mice. Vaccine viral loads were determined at in the injection site (muscle) at the time points indicated following vaccine administration to K18-hACE2 transgenic mice

Fig. 6

Comparison of VSV-WT with rVSV-ΔG-SARS-CoV-2-S vaccine effects following i.c. administration to C57BL/6 J or IFNAR KO mice. a C57BL/6 J mice, 6–12 weeks of age, administered i.c. with VSV-WT or rVSV-ΔG-SARS-CoV-2-S vaccine virus at doses ranging from 10³ to 10⁵ PFU (n = 3/group). b IFNAR KO mice, 6–12 weeks of age, administered i.c. with VSV-WT or rVSV-ΔG-SARS-CoV-2-S vaccine virus doses ranging from 10² to 10⁴ PFU. VSV-WT-injected mice were censored at day 2/3 due to lethality, while rVSV-ΔG-SARS-CoV-2-S-injected mice survived throughout the follow-up period (14 days)

Fig. 7

Histopathology of C57/BL6 mouse brain after injection with rVSV-ΔG-SARS-CoV-2-S. a Frontal cortex; b Striatum; c Thalamus; d Hypothalamus; e Hippocampus; f Cerebellum. Scale bar = 100 µm, Magnification: X10, H&E staining