ChAdOx1 nCoV-19 (AZD1222) vaccine candidate significantly reduces SARS-CoV-2 shedding in ferrets

Abstract

Vaccines against SARS-CoV-2 are likely to be critical in the management of the ongoing pandemic. A number of candidates are in Phase III human clinical trials, including ChAdOx1 nCoV-19 (AZD1222), a replication-deficient chimpanzee adenovirus-vectored vaccine candidate. In preclinical trials, the efficacy of ChAdOx1 nCoV-19 against SARS-CoV-2 challenge was evaluated in a ferret model of infection. Groups of ferrets received either prime-only or prime-boost administration of ChAdOx1 nCoV-19 via the intramuscular or intranasal route. All ChAdOx1 nCoV-19 administration combinations resulted in significant reductions in viral loads in nasal-wash and oral swab samples. No vaccine-associated adverse events were observed associated with the ChAdOx1 nCoV-19 candidate, with the data from this study suggesting it could be an effective and safe vaccine against COVID-19. Our study also indicates the potential for intranasal administration as a way to further improve the efficacy of this leading vaccine candidate.

Fig. 1. Study Outline.

Groups of eight ferrets (four male, four female) received either a single (prime) or two (prime-boost) doses of ChAdOx1 nCoV-19 via the intramuscular or intranasal route. An additional four ferrets (two male, two female) were included as mock (PBS) vaccinated control animals. Samples were collected from all of the animals at the timepoints highlighted.

Fig. 2. Serum virus-neutralisation titres against SARS-CoV-2.

Individual ferret serum samples were tested using a virus microneutralisation assay with a twofold dilution series starting at 1:10, with the neutralisation titre stated as the highest dilution of serum leading to 100% virus neutralisation. a Mean titres were calculated for each study group from log₂-transformed data and plotted with SEM. Green vertical lines represent dates of ChAdOx1 nCoV-19 administration, whilst red vertical lines represent virus challenge at study day 28 or 56 for the two cohorts, respectively. The dark green horizontal line represents the limit of detection of the assay. b Neutralising titres for individual ferrets were presented in a heatmap with vaccine administration and challenge dates represented with green and red lines, respectively. c Integrated analysis of neutralisation titres from prime-only and prime-boost administrations. Neutralisation titres from prime-only animals were combined from study days 7, 14, 24 and 40, whilst neutralisation titres from prime-boost animals were combined from study days 35, 42, 52 and 70. These represent the same timeframe relative to the challenge date. For both intramuscular and intranasal administration of ChAdOx1 nCoV-19, the increase in neutralisation titre from a boost dose is similar, as indicated by the almost-parallel linking lines. The vertical lines with error bars represent the mean and standard error of the mean (SEM) from log₂-transformed data.

Fig. 3. Comparative neutralisation of D614- and G614-containing isolates.

Six serum samples from prime-boost animals were selected from study day 35 and 42 samples per administration route. Microneutralisation assays were performed for each sample using three Australian virus isolates containing D614 (VIC01, SA01) or G614 (VIC31) in the Spike protein. Triplicate titres were obtained for each serum/isolate combination, with log₂-transformed mean values plotted as individual data points. Mean titres were calculated for each isolate/route of administration based upon log₂-transformed data, which were plotted as blue or red lines, depending on the route of administration. Error bars represent SEM, **P < 0.01.

Fig. 4. IFNγ-producing PBMC levels pre- and post challenge.

PBMCs were isolated from blood samples taken on days −4 and 5, relative to challenge, for quantification of circulating IFNγ-producing cells by ELISpot assay. Prime-only animals had few IFNγ-producing cells, whilst prime-boost animals had greater numbers. Comparison of intramuscular prime-boost with prime-only post-challenge results demonstrated statistical significance (**P < 0.01). Horizontal lines represent mean and SEM.

Fig. 5. Viral load in nasal-wash and rectal and oral swab fluids as determined by RT-qPCR.

RNA was extracted from samples collected on days 3, 5, 7 and 9 post challenge (study days 31, 33, 35 and 37 for prime-only animals; days 59, 61, 63 and 65 for prime-boost animals), and was analysed in duplicate using RT-qPCR assay detecting SARS-CoV-2 E RNA. Black points represent mean RNA copy number of the duplicate reactions for individual ferrets. The columns represent mean RNA copy numbers for each group, error bars represent SEM and the green lines are the limit-of-detection for the assay. Where no viral RNA was detected by RT-qPCR, a data point has been plotted at 3.9. Statistical analysis was performed by one-tailed t test, with statistical significance indicated by an asterisk (P < 0.05).