Inadequate use of masks or respirators put U.S. healthcare workers at risk of 2009 H1N1 flu during the earliest stages of the 2009 pandemic, researchers say.The study was led by researchers at the Centers for Disease Control and Prevention in Atlanta, who tracked 63 Southern California healthcare workers who had contact with six of the first eight laboratory-confirmed 2009 H1N1 cases detected in the United States.Because these contacts happened before the 2009 H1N1 outbreak had been widely reported, the cases shed light on how well healthcare workers protect themselves before a direct epidemiological threat becomes evident, the researchers said.The study, published in the journal Infection Control and Hospital Epidemiology, found nine of the healthcare workers tracked became infected with the 2009 H1N1 virus -- likely from contact with infected patients. Twenty of the 63 healthcare workers reported they had worn a mask or respirator at least once when in contact with patients, and no one from that group became infected.In addition, 43 workers reported never using a mask around patients, and all nine infections occurred in workers from this group, the study found.Overall, mask and respirator use was disappointingly low and only 19 percent reported using a mask during every patient encounter, the researchers said.