Even as public health data on the new swine flu outbreak pour in, the biological mechanism by which the virus sickens people remains poorly understood, scientists say.

Forty cases of swine flu have been confirmed in five U.S. states, says Richard Besser, acting director of the Centers for Disease Control and Prevention in Atlanta. But only one U.S. case has led to hospitalization, and no one in the United States has died. The cases appeared in New York, Texas, Kansas, Ohio and California, Besser said at a news briefing on April 27.

In contrast, hundreds of people have reportedly fallen ill in Mexico and a reported 149 have died from what is apparently the same strain of swine flu. Some reports put the mortality number much higher.

Also on Monday, reports emerged suggesting that swine flu cases have turned up in Britain, Canada, New Zealand and France.

The CDC advised against nonessential travel to Mexico and has taken the precautionary measure of making one-fourth of its strategic stockpile of antiviral flu drugs, or 11 million courses, available. But the drugs must be taken within the first 48 hours of the appearance of symptoms to be effective.

Meanwhile, the virus causing this concern remains something of a mystery.

“What are the biological properties that this virus really possesses when it comes to causing human disease?” asks Andrew Pekosz, a virologist at Johns Hopkins University in Baltimore. With the starkly different death reports in Mexico and the United States, he says, “we’re getting two different stories on how dangerous it is.”

Scientists have recently deciphered the virus’s main components and found that it contains genetic material from swine, birds and humans. But it remains unknown how this combination would render it particularly lethal, if it does.

Every influenza virus has on its surface two types of proteins, hemagglutinin and neuraminidase, which also occur in different varieties designated by number. The current swine flu is caused by an H1N1 virus, and an H1N1 is common among flu in people. But the hemagglutinin protein typically seen in human flu viruses is a human version, whereas the H1 in the current swine flu comes from a pig flu strain. The virus therefore might cause a strong reaction in the human body and severe symptoms, Pekosz speculates.

Scott Wetterhall, an epidemiologist at the Atlanta office of RTI International, a research institute, says that until the biological properties of the virus are better understood, public health officials faced with an outbreak will turn to four actions to mitigate it, some of which Mexico has taken. These include limiting interpersonal contact, closing schools, canceling social events and quarantining people who are ill or suspected of being exposed.

There is hope that the timing of the outbreak might limit its impact. “We’re nearing the end of the season in which flu viruses tend to transmit very easily,” Besser said at the briefing. But he also sounded a somber note: “We don’t think any of the existing vaccines are effective” against this strain. “I want you to understand, we view this as a marathon. We do think that this will continue to spread.”

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