SETH BORENSTEIN,AP Science Writer

WASHINGTON (AP) ó The way swine flu multiplies in the respiratory system is more severe than ordinary winter flu, a new study in animals finds.

Tests in monkeys, mice and ferrets show that the swine flu thrives in greater numbers all over the respiratory system, including the lungs, and causes lesions, instead of staying in the nose and throat like seasonal flu.

In addition, blood tests show that many people who were born before the 1918 flu pandemic seem to have immunity to the current swine flu, but not to the seasonal flu that hits every year.

The research by a top University of Wisconsin flu researcher was released Monday and will be published in the journal Nature.

“I’m very concerned because clearly the (swine flu) virus is different from seasonal influenza,” said study lead author Yoshishiro Kawaoka. “It’s a lot more severe.”

But it is still not as severe as the 1918 influenza, he said.

With only a few months since swine flu was first identified, doctors are still trying to get a handle on this flu strain and how it is different from the yearly seasonal flu.

The latest study paints a more pessimistic picture of the flu’s strength and the vulnerability of the elderly than how federal health officials have been portraying the situation.

Officials at the Centers for Disease Control and Prevention said Monday swine flu is acting differently than seasonal flu and they aren’t comparing its virulence to the run-of-the-mill influenza, which kills about 36,000 Americans per year. The CDC had no immediate comment on the Nature study.

Unlike seasonal flu, the new swine flu is continuing into the summer, and has caused severe illness mostly in younger people instead of the elderly, the CDC said.

The CDC’s Dr. Anne Schuchat said late last month that people over 65, and maybe people over 50 “are less likely to get ill with this virus even when they’re in a family with somebody who has it.”

A CDC study in May also found that one-third of senior citizens had some immunity to swine flu.

But Kawaoka did not find that. He checked blood samples from a wide number of age groups. With two exceptions, he found only people who were born before the 1918 pandemic to have immunity.

W. Paul Glezen, a flu epidemiologist at the Baylor College of Medicine in Houston who was not part of the study, said he would tend to agree with the earlier CDC study on immunity, especially since current figures show younger people sicker.

But Glezen also agreed with Kawaoka that the swine flu “appears to be more virulent than the seasonal” flu.

For his study, Kawaoka tested three monkeys with swine flu and three with seasonal flu. His data showed that there was at least twice as much virus in all parts of the lungs, the tonsils, windpipe, and nose for the swine flu-infected monkeys.