Two out of three people who died or were sent to hospital with swine flu in the spring were female, Canadian researchers say. (CBC)

Many people who were sent to hospital with the H1N1 virus in the United States had no underlying medical condition, U.S. health officials said Tuesday.

The U.S. Centers for Disease Control and Prevention looked at 1,400 adults and 500 children who were sent to hospital with swine flu in 10 major U.S. cities to try to understand who is or isn't being hit hardest by the virus.

"The vast majority of hospitalizations and deaths are occurring in younger people," spokesman Dr. Anne Schuchat told reporters in a telephone briefing.

In 46 per cent of the cases, the patients had no underlying medical problems.

In adults, the most common underlying conditions were asthma and chronic lung disease, chronic heart disease and weakened immune system, Schuchat said.

Among children, the most common underlying conditions were asthma and chronic lung disease, neurological or neuromuscular diseases, and sickle cell or other blood disorders, she added.

The CDC had not listed sickle cell disease before as a risk factor for severe disease from H1N1, but such children are normally considered a priority group for seasonal flu shots.

Earlier, health officials had said most people severely affected by swine flu had underlying conditions, but the new data suggest that may not be as common as once thought.

In the latest study, six per cent of the cases were pregnant women, who make up about one per cent of the general population in the U.S.

Similar Canadian data

The data is preliminary, CDC officials said at a news conference in Atlanta on Tuesday.

Obesity was not classified as an underlying condition in this study, nor was there a gender breakdown on the cases.

On Monday, Canadian researchers reported that 67 per cent of the people who died or were sent to hospital with swine flu in the spring were female.

About 30 per cent of the Canadian patients had a major underlying medical condition such as congestive heart failure, chronic renal disease and liver disease.

In the Canadian study, nearly 23 per cent of the patients had asthma, and 33 per cent were obese.

In most cases, people who get sick with swine flu stay at home and get better on their own.

In another development, the U.S. has made 9.8 million doses of H1N1 vaccine available as of Monday, but many Americans who want the vaccine may not be able to receive until the end of the month, when Canadian public health officials also plan to roll out their swine flu vaccination campaign.

American states need to place orders, and the vaccine needs to be packaged and shipped to offices and clinics.

"Even if more vaccine isn't available until November or December, we think it will be just fine to be vaccinated then," Schuchat said.

Mandatory injections questioned

In New York State, two nurses have filed lawsuits over mandatory H1N1 vaccinations, saying it hasn't been proven safe and effective.

New York health officials said they have the authority to make the vaccine mandatory in order to protect the public's health, and that the nurses could be fired if they don't get the shot.

New York is unique in North America in demanding that doctors, nurses and hospital workers be vaccinated to ensure those treating the infected stay healthy themselves.

In the spring, New York City was among the world's most affected regions. This fall, as flu season picks up elsewhere in the U.S., reported cases of flu in the city are down, another sign of the unpredictability of the virus, heath officials say.

In other jurisdictions such as most Canadian provinces and U.S. states, health workers are strongly encouraged, but not forced, to get the shot.

On Tuesday, Bob McCormack, chief medical officer for Canada's Olympic team, told CBC News that Olympic athletes should receive the swine flu vaccine on a priority basis to prevent an outbreak at the Winter Games in Vancouver.

With files from The Associated Press