12.4.2.2

Adenovirus Vectored COVID-19 Vaccines: Design,

Manufacturing, and Efficacy

One such example of a vaccine utilizing a chimpanzee adenoviral vector is the

Oxford/AstraZeneca vaccine (OA), also known as ChAdOx1 nCoV-19; AZD-1222.

It contains a DNA sequence coding for a full length, codon optimized S protein. The

European Medicines Agency assessment report on the OA vaccine describes

the finished product as a multidose suspension for injection containing ≥2.5 × 10⁸

infectious units of ChAdOx1 adenovirus vector per 0.5 mL dose [48]. The antigen is

the S-protein fused to the tPA leader sequence using a modified CMV promoter.

The vector is described as derived from chimpanzee adenovirus Y25 that was

rendered replication-deficient through the deletion of E1. It also includes deletion of

E3 and substitution of ORFs from human Ad5. The viral vector is propagated in a

derivative of HEK-293 cells known as T-Rex-293. This cell line contains an E1

Ad5 gene stably integrated into chromosome 19, making this cell line ideal for the

propagation of E1-deleted replication deficient adenoviruses.

The manufacturing process is divided into the two conventional steps to produce

a biopharmaceutic: upstream and downstream processing. For the upstream phase,

the cell culture is expanded from a pre-inoculum in shake flasks and rocker bags

which are used to seed a bioreactor for further expansion. This inoculum is then

transferred to the production bioreactor to generate the crude AZD-1222 active

principle. The cell culture in the bioreactor is then lysed, treated with nuclease to

degrade host-cell DNA and then sterile filtered. Chromatography and concentration

steps followed by diafiltration to remove any remaining impurities are well estab-

lished steps in the generic downstream process of adenoviral vectors. Finally, there

is a formulation step followed by freezing the final product at −55 to −90°C. For

more details of the adenovirus vectored-vaccines manufacturing, see Chapter 11.

The OA vaccine was originally designed as a single dose vaccine, but due to

lower protection than expected (43% and 80% reduction in risk of emergent hos-

pitalization and severe infection, respectively), a booster dose was suggested

within 4–12 weeks of the initial dose. This booster dose resulted in an efficacy of

70.4% [49]. Clinical trials also demonstrated good safety results [50]. However,

vaccine rollout was notably temporarily ceased in March 2021 after reports of

thromboembolic events including several unexpected deaths. Among the 5 million

vaccine recipients at the time, there were 30 cases of thrombotic events, mostly

venous thromboembolisms that were believed to have occurred due to the gen-

eration of antibodies against platelet factor 4 resulting in a vaccine induced

thrombotic thrombocytopenia similar to a heparin induced thrombocytopenia [51].

Another adenoviral vector vaccine approved for emergency use against COVID-

19 is the Johnson & Johnson vaccine (J&J), also known as Ad26.COV2-S. The

vaccine uses an adenovirus serotype 26 as the vector and is administered as a single

dose. Phase 3 clinical trials demonstrated 66% and 85% protection against moderate

and severe disease, respectively, and 100% efficacy against COVID-19 induced

hospitalization and death [52,53].

One example of a heterologous vaccine approach is the Russian Sputnik V, also

known as Gam-COVID-Vac. They use two recombinant adenovirus vectors (Ad5

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Bioprocessing of Viral Vaccines