Nov. 3 (HealthDay News) -- Californians both young and old are contracting H1N1, with infants the most likely to be hospitalized and adults over the age of 50, once hospitalized, most likely to die, a new study shows.

The report, appearing in the Nov. 4 issue of the Journal of the American Medical Association, somewhat contradicts the popularly held notion that elderly people are relatively immune from the ravages of this new infection.

Even though the numbers of elderly hospitalized and dying are relatively small, "that small proportion that are hospitalized who are elderly should be watched because they are at a higher risk of having a bad outcome once hospitalized. It doesn't mean they have a higher risk of infection," said Dr. Janice Louie, lead author of the study and a public health medical officer with the California Department of Health.

One federal expert agreed. "This is quite consistent with data we've seen from this country as well as globally and it does emphasize that providers should think of H1N1 in all age groups," Dr. Thomas Frieden, director of the U.S. Centers for Disease Control and Prevention, said at a Tuesday teleconference. "It doesn't change what the recommendations would be for vaccination because H1N1 is overwhelmingly affecting those under the age of 65," he added.

The first U.S. cases of H1N1 appeared in California on April 17, 2009, then spread rapidly.

Data so far indicates that infection with H1N1 results in relatively mild illness and that most people who become ill are younger: 40 percent are aged 10 to 18 and 95 percent are under the age of 50. Experts have longer thought that many older Americans may have some level of immunity to H1N1, because of exposures to prior outbreaks decades ago.

In the study, the team looked at the first 1,088 H1N1 cases in California that involved hospitalization or death between April 23 and Aug. 11, 2009.

The median age of patients was 27 and about a third were children under the age of 18, the researchers report.

"This implies that a good proportion of cases that were hospitalized were young adults, which is consistent with other findings," Louie said.

However, while infants had the highest rate of hospitalization, people over 50 were most likely to die once they ended up in the hospital.

"We had 118 who died, and most of those were adults. Only eight children died," Louie said.

Eleven percent of patients hospitalized died, many of them from viral pneumonia.

More than two-thirds (68 percent) of the patients had other conditions, including obesity, that put them at higher risk for more severe illness due to the flu.

"If you look at the [H1N1] data worldwide, the elderly do tend to have a higher risk of death and that's probably associated with risk factors because as you get older, you tend to accumulate things you don't necessarily want, like heart disease and lung disease," Louie said.

"This is really along the lines of what we've been seeing," agreed Dr. Edward Walsh, a professor of medicine at the University of Rochester Medical Center and chief of infectious diseases at Rochester General Hospital. "A relatively small number of older adults are infected but when they do get infected, they end up in the hospital at higher rates and they're dying at higher rates. Younger people are, by and large, a pretty sturdy group. The only difference is pregnant women [20 percent of hospitalized pregnant women needed intensive care] and they saw that as well."

Another expert also wasn't surprised by the finding.

"Clearly there are fewer elderly people getting this disease but it shouldn't surprise anybody that if an elderly person gets it and lands in the hospital they would have a higher death rate because it's a much frailer group. Twenty percent mortality in elderly people vs. 11 percent in young people -- I don't think that's shocking," said Dr. Jeffrey Boscamp, physician-in-chief at the Sanzari Children's Hospital at Hackensack University Medical Center in New Jersey.

A high proportion (about a third) of the California patients needed intensive care, the researchers found, and one-third of affected children needed ventilators.

The patterns seen here (and elsewhere) are markedly different from trends seen with the seasonal flu, where hospitalizations and deaths are disproportionately seen in older people, the authors stated.

More than one-third of adults with H1N1 in this group had gastrointestinal symptoms such as nausea, vomiting or diarrhea, vs. only about 5 percent with the seasonal flu, the team said

In related news, a research letter in the same issue of JAMA reports a case of rhabdomyolysis -- a breakdown in muscle tissue -- in a 28-year-old woman who contracted H1N1 in June of this year. Doctors at Stanford University Medical Center say that -- like many at-risk patients -- this woman was obese. She also had a family history of sickle cell anemia and had smoked two or three cigarettes a day for the past year.

Rhabdomyolysis has been associated with different strains of flu and now, the authors stated, doctors should be on the lookout for this possible complication in patients with H1N1 as well. The patient in question became very sick and required intensive care but has since recovered.

SOURCES: Janice K. Louie, M.D., public health medical officer, California Department of Public Health; Edward Walsh, M.D., professor, medicine, University of Rochester Medical Center, and chief, infectious diseases, Rochester General Hospital, New York; Jeffrey Boscamp, M.D., physician-in-chief, Sanzari Children's Hospital, Hackensack University Medical Center, New Jersey; Nov. 3, 2009, press teleconference, with Thomas Frieden, M.D., director, U.S. Centers for Disease Control and Prevention; Nov. 4, 2009, Journal of the American Medical Association