from IRS website: Foreign scholars, teachers, researchers, or trainees who arrive in the United States in O-1 status or TN status (from Canada or Mexico under the NAFTA treaty) are fully liable for U.S. social security/Medicare taxes if they are employed on the payroll of the university or other employer, regardless of whether or not they are resident or nonresident aliens unless the provisions of a Totalization Agreement relieve such aliens from liability for U.S. social security/Medicare taxes