As others have pointed out, the vaccine studies seek to include a wide range of people and then look for tenancies within the data that might suggest that people sharing certain characteristics (e.g. ethnicity, gender, medications, environments, medical histories, etc.) might have better or poorer results from the vaccine. Do you really expect (and would you really support) studies that focused on narrow groups? And how many of these narrow studies would we have to run? Is a study needed for left-handed Irishmen with green eyes?
Regardless of the above, I agree that the pregnant nurse has every right to refuse the vaccination. It is her decision to make.
That said, she has to be prepared to accept the consequences of her decision. As with any other employer, the hospital is within its rights to insist that employees do things that are needed to competently and safely do their jobs. People unwilling or unable to do these things can't expect to be employees.
The whole concept of "reasonable accommodation" comes from the Americans with Disabilities Act and IIRC only applies to the physical and mental disabilities identified in the Act.
http://http://en.wikipedia.org/wiki/Americans_with_Disabilities_Act_of_1990
The pregnant nurse's decision to not be vaccinated is NOT a disability.
Similarly, she's entitled to decide that even being in the hospital with its attendant exposure to sicknesses poses an unacceptable risk to her pregnancy. She might also conclude that the electromagnetic fields generated by medical equipment is an unacceptable risk to her baby. She can decide for any reason she wants that what the hospital requires is not something she can accept because of her pregnancy.
The hospital is NOT responsible for proving to her satisfaction that she is mistaken, or that their (assumedly legal) requirements of employees are reasonable. It is her decision to either "take them or leave them".