ATLANTA (AP) -- Swine flu was four times more likely to send blacks and Hispanics to the hospital than whites, according to a study in Chicago that offers one of the first looks at how the virus has affected different racial groups.

The report echoes some unpublished information from Boston that found three out of four Bostonians hospitalized from swine flu were black or Hispanic.

The cause for the difference is probably not genetic, health officials said. More likely, it's because blacks and Hispanics suffer disproportionately from asthma, diabetes and other health problems that make people more vulnerable to the flu.

It's not clear if a racial or ethnic difference will hold up when more complete national data is available, one federal health official said. The findings are based on fairly small numbers of cases from the early days of the pandemic.

"We don't have anything definitive to say one group is more affected than another," said Dr. Daniel Jernigan of the U.S. Centers for Disease Control and Prevention.

The Chicago findings, released Thursday, are believed to be the first published study to detail a racial or ethnic breakdown of swine flu's impact.

Researchers looked at more than 1,500 lab-confirmed swine flu cases reported to the Chicago Department of Public Health from late April through late July.

Blacks with swine flu were hospitalized at a rate of 9 per 100,000, and Hispanics at a rate of 8 per 100,000. For whites, the rate was 2 per 100,000, the study found.

Earlier this month, Boston health officials released some unpublished information that found three out of four Bostonians hospitalized with swine flu were black or Hispanic.

"It's very disturbing," said Barbara Ferrer of the Boston Public Health Commission, speaking about the higher rates of minority swine flu hospitalizations.

"But intuitively it's understandable, because we have tremendous inequities in most areas of health," said Ferrer, the agency's executive director.

Also, experts noted that the Chicago and Boston data represent limited information from only two cities and only the first two or three months of the pandemic. The unpredictable manner of swine flu outbreaks means some parts of the city were hit before others -- a sequence that may have little to do with race.

"I think it reflected more the neighborhoods the disease was first going through," said Jernigan, a CDC flu expert.

This fall, the government will be doing national surveys to better track swine flu trends. That should provide more reliable information about how the virus is affecting different groups of people, he said.

Copyright 2009 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.