Using an attenuated virus Dr. Hextall identifies a possible vaccine. To cut out the lengthy time it would take to obtain informed consent from infected patients, Dr. Hextall inoculates herself with the experimental vaccine and immediately visits her gravely ill father (Dan Flannery), who has been infected with MEV-1. The doctor does not contract MEV-1 and the vaccine is declared a success.

Who is used to test experimental vaccines in contagion? Let's have some stream of consciousness first.
The relevant information to answer the above question is: To cut out the lengthy time it would take to obtain informed consent from infected patients, Dr. Hextall inoculates herself with the experimental vaccine and immediately visits her gravely ill father (Dan Flannery), who has been infected with MEV-1. So, the answer is Dr. Hextall.