Tiny infants are most likely to be hospitalized with swine flu, but people over the age of 50 are most at risk of dying in hospital from the disease, US researchers said Tuesday.
"Despite having lower hospitalization rates, persons aged 50 years or older who were hospitalized with pandemic 2009 influenza (A)H1N1 infection were among those most likely to die," a study conducted by researchers at the California department of health showed.
Around 11 percent of the 1,088 cases of swine flu reported in California between April 17 and August 22 died, the study showed.
The death rate for people 50 and older was around 19 percent, compared to seven percent for children under 18, who made up around a third of swine flu cases reported in California.
Nearly 12 infants out of 100,000 had to be hospitalized for swine flu in the first four months of the outbreak in California, giving the tiny tots the highest hospitalization rate of any age group.
"The youngest age group, infants aged two months or younger, had the highest hospitalization rates but are too young to receive currently licensed influenza vaccines," the study said.
The findings support recommendations for caregivers of the very young to be given priority for swine flu vaccination, it added.
US health officials have repeatedly characterized swine flu as "a younger person's disease," and put children and young adults under the age of 25 on a list of five priority groups for vaccination against the disease.
The others are health care workers, infant caregivers, pregnant women and adults up to age 65 with certain underlying health conditions.
The California researchers said the median age for hospitalized and fatal swine flu cases was 27 years -- younger than is commonly seen with seasonal flu.
"For seasonal influenza, persons older than 64 years, younger than five years, or who have specific medical conditions have higher rates of hospitalization and death," the study said.
Even though swine flu is perceived as causing "only mild disease," the researchers also found that about a third of hospitalized cases were severely ill and required intensive care.
Twenty percent of pregnant women in the study spent time in the intensive care unit, and most hospitalized adults, and more than one-third of children, required mechanical ventilation.
The most common causes of death were viral pneumonia and acute respiratory distress syndrome, the study said.
The United States is battling swine flu amid shortages of vaccine, which has seen states and counties cancel inoculation clinics, and of children's anti-viral medicine, which last week saw federal authorities raiding the strategic stockpile for kids' liquid Tamiflu.
Pandemic H1N1 flu has already claimed the lives of more children than seasonal flu typically does during an entire flu season, which runs from August until May.
More than 5,700 people have died worldwide since the virus was first discovered in April, with most of the deaths -- 4,175 -- in the Americas region, the World Health Organization said Friday.