N95 masks need to be adjusted for each person. (Nathan Denette/Canadian Press)

About two-thirds of the children killed in the U.S. by the H1N1 swine flu virus had underlying medical conditions, the U.S. Centers for Disease Control and Prevention reported Thursday.

As of Aug. 8, CDC had received reports of 477 deaths associated with 2009 pandemic influenza A (H1N1) in the United States, including 36 deaths among children under the age of 18.

"The results of that analysis indicated that, of 36 children who died, seven (19 per cent) were aged under five years, and 24 (67 per cent) had one or more of the high-risk medical conditions," researchers wrote in the CDC's Morbidity and Mortality Weekly Report.

Medical conditions that seemed to put children at increased risk of severe disease from the H1N1 virus included asthma and disabilities such as cerebral palsy or muscular dystrophy.

In 22 per cent of the cases among children under five, the victims were perfectly healthy before they fell ill with swine flu.

Bacterial infections on top of H1N1

Some children who didn't have underlying conditions were infected with bacteria. Given that finding, doctors should keep in mind that if a child is sick with flu, gets better and then spikes a fever, antibiotics may be needed, said CDC director Dr. Thomas Friedman.

Yet of the hundreds of thousands of people infected in New York, the overwhelming majority were moderate illnesses that didn't need testing or treatment. It's people who have severe illness such as trouble breathing or keeping fluids down, or those have underlying illnesses who should seek prompt treatment, Friedman told a news conference in Atlanta.

Antiviral medications work best if given in the first 48 hours but anyone with serious illness should be treated, he added.

Regulators are also watching closely for any increase in the number of adverse events associated with swine flu vaccines worldwide beyond baseline levels. For example, if 500,000 pregnant women were vaccinated with a placebo, more than 1,000 miscarriages would be expected in the week following the shot normally, Friedman said in describing the baseline.

N95 respirators for health-care workers

Also on Thursday, the U.S. Institute of Medicine released a report recommending that doctors, nurses and other health-care workers caring for people infected with H1N1 should wear N95 respirator masks rather than surgical masks to protect themselves.

During the 2003 SARS outbreak that killed 44 people in the Toronto area, nurses in Ontario said they lacked N95 respirators that were properly fitted. Personal protective equipment such as gloves and gowns are also recommended for infection control purposes.

While much is still not known about how influenza viruses are transmitted, wearing a respirator on top of other hygiene measures like handwashing is the best defence, the report's authors said.

It is still unclear what proportion of influenza viruses are spread by contact, droplet spray or airborne, and how masks block each of these modes of transmission, according to the report.