unknown to me.

Claimed to be doing a phone survey for the Center for Disease Control concerning the immunization history of cell phone users.

First time they called, I answered. Got a recorded message saying they wanted to take a survey. I hung up. They continue to call. I repeatedly hang up or I just dont answer. Finally filed a complaint with the do not call list.

blocked it but they keep trying to call so my phone vibrates

they seem to be legitimate.  Caller ID "NORC U CHICAGO" matches information on the CDC website.  

from http://www.cdc.gov/NIS/faq_h1n1.htm
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Who is NORC?
NORC, the National Opinion Research Center, is a not-for-profit research organization at the University of Chicago. NORC has been conducting social science research for over 60 years and has been contracted to conduct the National 2009 H1N1 Flu Survey for the Centers for Disease Control and Prevention.