From Nerruan-Webster:
Definition of vaccine

1 : a preparation that is administered (as by injection) to stimulate the body's immune response against a specific infectious agent or disease... [merriam-webster.com]

There is no guarantee that a vaccine will provide 100% immunity. A vaccine is simply a method to stimulate the body's immune system to produce antibodies that will fight a viral infection. How long that response will last varies. The COVID vaccines were never intended to keep people from contracting the virus, or even become infectious themselves. The vaccines were intended to help the body to fight the virus. Best case would be the vaccine prevented an exposed person to never become infectious and/or infected. Worst case would be an infection that causes a mild case of COVID, but prevents the exposed person from becoming critically ill.

The AZ antibody injection is using someone else's antibodies. It is intended for those who won't produce antibodies on their own due to compromised immune systems. It also is much more involved that an IM injection or 2. Antibody treatments like this are a subsitute for those who can't produce the antibodies themselves. See the difference?

[nature.com]
Your question makes no sense since vaccines do prevent disease but this does explain the difference between therapies.

If a vaccine works, it is therapeutic?

Pfizer-BioNTech and Moderna
DNA and RNA vaccines use fragments of genetic material made in the lab. These fragments code for a part of the virus (such as its spike protein). After the vaccine is injected, your body uses instructions in the DNA/RNA to make copies of this virus part (or antigen). Your body recognises these and mounts an immune response, ready to protect you the next time you encounter the virus. mRNA vaccines do not use the live virus that causes COVID-19. They do not affect or interact with our DNA in any way. mRNA never enters the nucleus of the cell, which is where our DNA (genetic material) is kept. The cell breaks down and gets rid of the mRNA soon after it is finished using the instructions. Pfizer-BioNTech and Moderna are (messenger) mRNA vaccines.

Oxford/AstraZeneca
Virus vector vaccines use a virus, often weakened and incapable of causing disease itself, to deliver a virus antigen into the body. It is made from a weakened version of a common cold virus (known as an adenovirus) from chimpanzees. It has been modified to contain genetic material shared by the coronavirus - although it can't cause the illness. The virus’ ability to infect cells, express a large amount of antigens and in turn trigger a strong immune response is how the virus vector is effective. One high-profile example is the University of Oxford/AstraZeneca vaccine.
[theconversation.com]
[cdc.gov]

Let's look at specific definitions:

Monoclonal antibody therapy is a form of immunotherapy that uses monoclonal antibodies to bind monospecifically to certain cells or proteins. The objective is that this treatment will stimulate the patient's immune system to attack those cells.

Vaccine - a substance used to stimulate the production of antibodies and provide immunity against one or several diseases, prepared from the causative agent of a disease, its products, or a synthetic substitute, treated to act as an antigen without inducing the disease.
"every year the flu vaccine is modified to deal with new strains of the virus"