Younger people are at greater risk of catching swine flu, with most cases occurring in teenagers, the World Health Organization said.

The median age of those infected with the pandemic H1N1 virus is 12 to 17 years, WHO said in a statement yesterday, citing data from Canada, Chile, Japan, U.K. and U.S. Patients requiring hospitalization and those with fatal cases may be slightly older, the Geneva-based United Nations agency said.

“As the disease expands broadly into communities, the average age of the cases is appearing to increase slightly,” WHO said. “This may reflect the situation in many countries where the earliest cases often occurred as school outbreaks but later cases were occurring in the community.”

World health officials are trying to determine which groups are most likely to get severely ill so measures to best protect them can be taken. Drugmakers are developing vaccines to fight the scourge, which WHO says may result in 2 billion infections.

Cardiovascular and respiratory diseases, diabetes and cancer put people infected with the new H1N1 virus at greater risk of developing severe complications, the UN agency said. Asthma and other forms of respiratory disease have been consistently reported as underlying conditions associated with more severe illness in several countries, it said.

Obesity has also been reported as a risk factor, and there is mounting evidence that pregnant women are at higher risk for more severe symptoms, WHO said. Some minority populations may also be more vulnerable, “but the potential contributions of cultural, economic and social risk factors are not clear.”

Vaccine Supply

Humans trials of a pandemic vaccine began in Australia this week, helping regulators gauge the safety and efficacy of shots.

The most common way to make flu vaccine is by growing the virus in fertilizer chicken eggs. The virus is then extracted, purified and killed for injection into humans, prompting the immune system to generate antibodies that fend off any infection.

WHO said yesterday that the yields for pandemic vaccine viruses are 25 percent to 50 percent of those of normal seasonal flu viruses for some manufacturers.

A network of WHO collaborators is trying to develop higher- yielding vaccine virus candidates, the agency said, adding that it will be able to revise its estimate of pandemic vaccine supply once it has the new yield information.