Half of the doctors, nurses and other health workers who have become sick with the new H1N1 swine flu got it on the job, suggesting they are not taking enough care to protect themselves, U.S. health officials said on Thursday.

The World Health Organization has classified the H1N1 swine flu as a pandemic. So far, most infections have been mild, but the WHO has said the virus could change at any time to a more dangerous form.

Infections in the United States have begun to taper off, with hundreds of thousands infected.

The U.S. Centers for Disease Control and Prevention is taking stock of how the nation responded to the first wave of the virus, in anticipation of a second, possibly more serious, outbreak in the northern hemisphere autumn.

As of May 13, the CDC said it had received 48 reports of healthcare workers infected with swine flu.

They studied detailed case reports on 26, and found 13 were infected in a healthcare setting such as a clinic or hospital, and 12 caught it from infected patients, the CDC said in its weekly report on death and disease.

None of those who became ill said they fully followed the CDC's recommendations on infection control, such as wearing masks, surgical gloves, eye protection and gowns while caring for patients.

Doctors, nurses, technicians and other health workers becoming ill would be especially bad news during a pandemic, especially with a health system already struggling to handle overcrowded emergency departments and a nursing shortage.

These results highlight the need for healthcare facilities to maintain adherence to infection control recommendations, to recognize and triage potentially infectious patients, to provide adequate infection-control resources and to train staff in infection-control practices and proper use of personal protective equipment, the CDC team, with health officials from several states, wrote in the report.