CpG-adjuvanted stable prefusion SARS-CoV-2 spike protein protected hamsters from SARS-CoV-2 challenge

Abstract

The COVID-19 pandemic presents an unprecedented challenge to global public health. Rapid development and deployment of safe and effective vaccines are imperative to control the pandemic. In the current study, we applied our adjuvanted stable prefusion SARS-CoV-2 spike (S-2P)-based vaccine, MVC-COV1901, to hamster models to demonstrate immunogenicity and protection from virus challenge. Golden Syrian hamsters immunized intramuscularly with two injections of 1 µg or 5 µg of S-2P adjuvanted with CpG 1018 and aluminum hydroxide (alum) were challenged intranasally with SARS-CoV-2. Prior to virus challenge, the vaccine induced high levels of neutralizing antibodies with 10,000-fold higher IgG level and an average of 50-fold higher pseudovirus neutralizing titers in either dose groups than vehicle or adjuvant control groups. Six days after infection, vaccinated hamsters did not display any weight loss associated with infection and had significantly reduced lung pathology and most importantly, lung viral load levels were reduced to lower than detection limit compared to unvaccinated animals. Vaccination with either 1 μg or 5 μg of adjuvanted S-2P produced comparable immunogenicity and protection from infection. This study builds upon our previous results to support the clinical development of MVC-COV1901 as a safe, highly immunogenic, and protective COVID-19 vaccine.

Figure 1

Study design of the hamster challenge study. Hamsters were immunized twice at 3 weeks apart and 2 weeks after the second immunization, serum samples were taken for immunogenicity assays. Four weeks after the second immunization, hamsters were challenged with 10⁴ PFU of SARS-CoV-2. Body weights were tracked for 3–6 days after infection and the animals were euthanized on the third or sixth day after infection for necropsy and tissue sampling.

Figure 2

Neutralizing antibody titers with pseudovirus assay in hamsters 2 weeks after second immunization. Hamsters (N = 10 per group) were immunized twice at 3 weeks apart with vehicle control (PBS), 1 µg (LD) or 5 µg (HD) of S-2P adjuvanted with 150 µg CpG 1018 and 75 µg aluminum hydroxide, or with adjuvant alone. The antisera were harvested at 2 weeks after the second injection and subjected to (a) neutralization assay with pseudovirus expressing SARS-CoV-2 spike protein to determine the ID₉₀ titers of neutralization antibodies and (b) total anti-S IgG antibody titers with ELISA. Results are presented as geometric mean with error bars representing 95% confidence interval and statistical significance calculated with Kruskal–Wallis with corrected Dunn’s multiple comparisons test. Dotted lines represent lower and upper limits of detection (40 and 5120 in ID₉₀, 100 and 1,638,400 in IgG ELISA).

Figure 3

Change in body weight in hamsters after infection with SARS-CoV-2. Hamsters immunized in Fig. 2 were challenged with 10⁴ PFU virus. The body weights of individual hamsters were tracked daily up to the time of euthanizing at 3 d.p.i. (n = 5 per group) and 6 d.p.i. (n = 5 per group). Results are presented as mean with error bars representing standard error and statistical significance calculated with Two-way ANOVA with Tukey’s multiple comparison test at 3 d.p.i. (left) or 6 d.p.i. (right).

Figure 4

Viral load in hamsters 3 or 6 days post infection with SARS-CoV-2. The hamsters were euthanized at 3 or 6 d.p.i. and lung tissue samples were collected for viral load determination by (a). quantitative PCR of viral genome RNA, and (b). TCID₅₀ assay for virus titer. Results are presented as geometric mean with error bars representing 95% confidence interval and statistical significance calculated with Kruskal–Wallis with corrected Dunn’s multiple comparisons test. Dotted lines represent lower and limit of detection (100).

Figure 5

Lung pathology scoring in hamsters 3 or 6 days post infection with SARS-CoV-2. The hamsters were euthanized at 3 or 6 d.p.i. and lung tissue samples were collected for sectioning and staining. The histopathology sections were scored as outlined in the methods and the results tabulated. Results are presented as mean of lung pathology scores with error bars representing standard error and statistical significance calculated with one-way ANOVA with Tukey’s multiple comparisons test.