不付FICA的两个可能理由，一个是工作单位是州政府医院，有的州政府雇员参与州政府退休计划而不是SS，这个理由由于原贴主细节不清楚，无法验证。
第二个理由是，实习医生是学生，不是雇员，所以不用交FICA
第二个理由不成立，参见以下链接
https://www.irs.gov/pub/irs-regs/td_9167.pdf
Example 4. (i) Employee E is employed by University V to provide patient care services at a teaching hospital that is an unincorporated division of V. These services are performed as part of a medical residency program in a medical specialty sponsored by V. The residency program in which E participates is accredited by the Accreditation Counsel for Graduate Medical Education. Upon completion of the program, E will receive a certificate of completion, and be eligible to sit for an examination required to be certified by a recognized organization in the medical specialty. E’s normal work schedule, which includes services having an educational, instructional, or training aspect, is 40 hours or more per week.
(ii) In this example, E is employed by V, a school, college, or university within the meaning of paragraph (c) of this section. However, E’s normal work schedule calls for E to perform services 40 or more hours per week. E is therefore a full-time employee, and the fact that some of E’s services have an educational, instructional, or training aspect does not affect that conclusion. Thus, E’s services are not incident to and for the purpose of pursuing a course of study. Accordingly, E’s services are not excepted from employment under section 3121(b)(10) and there is no need to consider other relevant factors, such as whether E is a professional employee or whether E is eligible for employment benefits
