Suggested link to 1918 Pandemic Virus

May 20 (Bloomberg) -- Adults older than 52 may have been exposed to a virus that gives them some protection against swine flu, explaining why younger people account for the largest percentage of those hospitalized, a U.S. scientist said.

The virus that caused the 1918 Spanish flu pandemic circulated until a bird flu virus replaced it during the 1957 pandemic, Daniel Jernigan, deputy director of the Centers for Disease Control and Prevention influenza division, told reporters. Exposure to the earlier virus “may allow you to have some protection” to swine flu, Jernigan said today.

Swine flu, known formally as H1N1, has sickened more than 10,000 people in 41 countries, according to the World Health Organization. The swift spread of the disease in Japan has pushed WHO to consider declaring a pandemic, according to Hitoshi O****ani, former head of the agency’s Western Pacific region. Researchers have been trying to explain why more than 60 percent of the estimated 5,700 U.S. swine flu patients have been ages 5 to 25, Jernigan said.

“When we look at the number of cases that have been hospitalized, the median age of those being hospitalized is 19 years,” Jernigan said. “Those 19 to 49 are the largest chunk being hospitalized.”

Jernigan said about 18 percent of those sick enough to need hospital care are from 10 years old to 18 years old and another 11 percent are ages 5 to 9.

CDC will publish a weekly report tomorrow elaborating on the finding, Jernigan said. Studies published in the New England Journal of Medicine May 7 said that swine flu evolved from human viruses circulating in pigs for more than a decade and that older people may have some immunity because of exposure to similar viruses.

To contact the reporter on this story: Nicole Gaouette in Washington at ngaouette@bloomberg.net.