I can’t show you a statute, but a quick google search pulls up this:
http://www.hhs.gov/nvpo/law.htm
“Immigration immunization laws
Under new immigration laws passed in 1996 and in effect as of July 1, 1997, all individuals seeking permanent entry into the U.S. must prove that they have been inoculated against all vaccine-preventable diseases. This includes infants and children being brought into the country for international adoption.(12)”
http://travel.state.gov/visa/immigrants/info/info_1331.html
“United States immigration law requires immigrant visa applicants to obtain certain vaccinations (listed below) prior to the issuance of an immigrant visa. Panel physicians who conduct medical examinations of immigrant visa applicants are required to verify that immigrant visa applicants have met the vaccination requirements, or that it is medically inappropriate for the visa applicant to receive one or more of the listed vaccinations:
— Acellular pertussis
— Hepatitis A
— Hepatitis B
— Human papillomavirus (HPV)
— Influenza
— Influenza type b (Hib)
— Measles
— Meningococcal
— Mumps
— Pneumococcal
— Pertussis
— Polio
— Rotovirus
— Tetanus and diphtheria toxoids
— Varicella
— Zoster”
My guess is that it may have happened in a process similar to this one:
http://www.cdc.gov/ncidod/dq/laws_regs/fed_reg/vaccine/vaccine_factsheet.htm
Where’s rabies?