Senior service fellow:

"Are you a talented scientist engaged in public health research, studies, training, or investigations? If so, you may qualify for one of our many fellowship programs."
"Senior Service Fellows
Senior Service Fellow
Qualifications: Doctoral degree or foreign equivalent obtained from a recognized college or university and substantial post-doctoral experience in a scientific field related to CDC's mission"
http://www.cdc.gov/employment/menu_fellows.html

"Post-doctoral Research Fellowships"
"2-year program to conduct high-priority research in infectious diseases"
"2-year appointment with potential for extension to conduct research"
"2-year fellowship to conduct research"
"must have earned degree within 3 years of start date"
http://www.cdc.gov/Fellowships/PostDoc.html

Tout correspond :

"On appelle souvent chercheur postdoctoral (ou, improprement, postdoctorant ou encore chercheur postdoctorat, souvent en abrégé en « postdoc ») un chercheur titulaire d'une thèse de doctorat (généralement depuis moins de dix ans) engagé en contrat à durée déterminée dans un laboratoire de recherche."
http://fr.wikipedia.org/wiki/Chercheur_postdoctoral