Why are clinical trials so complicated?

First you have to understand the virus. This is usually done either with sequencing the virus (finding its structure) or finding how it is related to past virus studies.

Once you know about the structure you try to find how it gets into a cell. This may involve things like "mouse models" where you create a special kind of mouse and study what happens.

Once you know how it gets into the cell you need to figure out how it replicates. Viruses use different parts of the cell to replicate and sometimes bring their own machinery.

You also have to know how the body responds. Can the immune system recognize the virus? If it does, can it mark the virus (antibodies)? Can the immune cells disable the virus or does the virus disable the immune cells?

Can you figure out a chemical to attack the virus with something that stops it from entering the cell or reproducing in the cell?

Does this chemical kill mice? If not, does this chemical which works in a test tube also work in the body? If it works, does it work well enough to matter?

Assuming it works in mice, does it kill humanes (Phase 1 study with volunteers)

Assuming it doesn't kill people, does it do anything? (Phase 2 study)

Assuming it does something will it work for most people (Phas 3 study)

Assuming it works for most people can we manufacture it in quantity (7 billion doses)?

The whole process takes about 5 years.