Results of ELISA for detection of monkeypox virus—specific IgG in acute- and convalescent-phase serum specimens obtained from health care workers (HCWs) from 3 facilities in Indiana (clinic A and hospitals A and B) in 2003, according to smallpox vaccination status. The adjusted optical density is defined in Subjects and Methods. Blue squares, patients who did not receive smallpox vaccination; red diamonds, patients who received smallpox vaccination; yellow region, equivocal.