I think it would be great to add a hidden construction for the lore of the mod that can be activated or deactivated where he spawns dr hyde's notes or that only in that structure spawns a special zombie which contains the note that is extremely rare,or that there is a probability where a zombie has an annotated map of the location of the lab where those notes are,Or perhaps that on some radio frequency you find a pre-recorded message or several which the doctor speaks about his investigation and in one of those transmissions you can identify where he is or where he is, and that these transmissions, for example, start after 1 year of departure in the game or that is configurable. more than anything for him single player mode.

My suggestion would be as follows:

1. we can test the efficacy of the crafted vaccine on to the infected organ (since you are planning on using the infected organ to be useable as a crafting material for the vaccine) of course the result would be dependent on the level of first aid and number of successful attempt in creating an vaccine (up to be decided).

2. or we can test the crafted vaccine to our sample blood in syringe by looking and using the microscope of course the result would be dependent on the level of first aid

3. the vaccine that was crafted should have side effects like flu, cold, or loss of appetite etc.

4. I know this is too much but we can test the vaccine into the zombies itself (the lower the efficacy the greater the side effects like becoming tough etc.)