May 7 (Bloomberg) -- Swine flu evolved from human viruses circulating in pigs for more than a decade, a finding that may explain why people in their 30s and 40s are getting sicker than the elderly in the U.S. and Mexico, scientists said today.

Older people may have some immunity against swine flu because of exposure to similar viruses as long as 70 years ago, before the virus’s ancestors switched to infecting pigs, said Robert Belshe, a St. Louis University influenza expert, who wrote a commentary on studies published in the New England Journal of Medicine.

The percentage of people ages 30 to 44 who are hospitalized from swine flu is higher than in seasonal flu, according to the U.S. Centers for Disease Control and Prevention. That suggests younger adults may have less immune protection against swine flu than older people, Belshe said.

“They may have been infected with similar viruses in the 1930s and 1940s,” Belshe said today in a telephone interview. “Their exposure to seasonal viruses may be giving them some protection.”

More than 2,300 people in 24 countries have been confirmed infected with swine flu, which was first detected in Mexico. The World Health Organization, the United Nations’ Geneva-based health branch, has said that a worldwide flu pandemic is “imminent.”

Teenagers Targeted

Only 5 percent of U.S. patients with confirmed swine flu are 51 years old or older, researchers led by the Fatima Dawood, a CDC epidemiologist, said in one of the two studies published today. About 60 percent of cases are in people ages 18 and younger, they said in the study.

Researchers at the Atlanta-based agency are looking for an explanation about the high number of cases in teenagers, Dawood said today in a conference call with reporters. Investigations of outbreaks in schools may have led public health officials to find and diagnose cases in younger people, while those in older people may have been missed, she said.

“This is an evolving outbreak and we’re still learning how this virus works,” she said.

Patient reports also suggest the swine flu, which CDC researchers are now calling swine-origin influenza H1N1/A, causes more vomiting and diarrhea than standard seasonal flus, the CDC said. The agency is advising doctors to look for flu patients with gastrointestinal symptoms and have them tested for swine flu, Dawood said.

Unique Blend

CDC researchers are calling the strain a triple reassortant because it has genetic elements of bird, pig and human influenzas. Most viruses of this type seen in North America contain familiar combinations of genes, the researchers said.

The swine flu virus contains sets of genes that make neuraminidase, a viral protein involved in replication, and matrix, a structural protein, which had never been seen outside of Asia before, CDC researchers said in one of the studies. The strain’s sudden appearance may indicate a gap in flu surveillance, said Michael Shaw, a CDC microbiologist.

“A lot of researchers are going back to their freezers to see if there’s anything they’ve overlooked,” in earlier flu samples, he said today on the conference call.

Related Strains

Related versions of the same strain of swine flu that mainly circulated in pigs infected at least 11 people in the U.S. from 2005 through 2009, CDC researchers said in another study in the New England Journal of Medicine. Those patients suffered standard flu symptoms, such as fever and cough, and about 30 percent had some diarrhea, said the scientists, who were led by Vivek Shinde, a CDC influenza expert.

While most swine flu cases have been relatively mild, at least two patients infected with these earlier strains had very severe cases of flu, with respiratory failure and hospital stays of 19 days to 30 days. One theory is that the H1N1 virus, in evolving to spread more easily from human to human, may have lost some though not all of its virulent potential, said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases in Bethesda, Maryland.

“That’s a hypothesis at this time,” he said in a telephone interview. Swine flu “looks like no more than a seasonal flu, but it is evolving.”

Normal, seasonal flus tend to target very young children and the elderly for severe consequences, which can include pneumonia and death, said Lone Simonsen, a George Washington University flu researcher, who wrote a commentary in the same journal. Younger adults can normally fight off seasonal flu relatively well, she said.

Lessons of Past

Those trends changed during past pandemic outbreaks, such as in 1918, when the Spanish flu killed an estimated 50 million people worldwide, she said. Archived records from Copenhagen indicate that even in the early stages of that flu, young adults 20 years old to 40 years old were vulnerable, she said today in a telephone interview.

Later, more waves of the same pandemic struck the same age group, she said. These findings suggest that public health officials should consider prioritizing vaccine supplies for people who are most vulnerable in the first wave of infection, rather than the standard risk groups that include the very young and very old, she said.

“If you think something like the 1918 flu is coming again, you would have to rethink your priorities,” she said. “It would not be business as usual.”

To contact the reporter on this story: John Lauerman in Boston at jlauerman@bloomberg.net.

Last Updated: May 7, 2009 19:22 EDT