Close contacts were defi ned as anyone who ever came within 2 meters of an index case-patient without the use of effective personal protective equipment (PPE) (including masks and gloves, with or without gowns or goggles) during the presumed infectious period. Trained staff from local CDCs made the determinations on the basis of fi eld investigation. The relationships of close contacts to index case-patients were categorized as 1) spouses, 2) other household members, 3) nonrelated roommates, 4) contacts at workplace or school, 5) nonhousehold relatives, 6) passengers on the same fl ight, 7) friends, and 8) service persons met at public places. A close contact on an aircraft was defi ned as a passenger sitting within 3 rows in front and 3 rows behind the index case-patient.