Aug. 25 (Bloomberg) -- A global outbreak of lethal flu might kill 207,000 people in the U.S. and crowd hospitals with 700,000 patients, according to a government study that will be released tomorrow.

Pandemic flu, which last struck the U.S. in 1968-1969 when 34,000 people died of Spanish flu, would cause symptoms severe enough to see a doctor in as many as 42 million people, said the report from the U.S. Department of Health and Human Services.

Health officials are seeking to prevent a global outbreak such as the one in 1918 that killed about 500,000 people in the U.S. and as many as 20 million people worldwide. Researchers are concerned that a bird flu virus that has killed eight people in Thailand and 20 in Vietnam this year will mutate into a strain with the potential to spread widely among people.

``Recent studies suggest that avian strains are becoming more capable of causing severe disease,´´ the study said. Should the strains become mixed with ``human influenza viruses such that they can be effectively transmitted between people, a pandemic can occur.´´

More than 100 million poultry have been killed in Asia in the past year to stem the spread of bird flu. Chiron Corp. and Aventis SA, the two biggest makers of flu vaccine for the U.S., are developing bird flu vaccines, as are research teams at the U.S. Centers for Disease Control and Prevention, based in Atlanta, and at St. Jude Children´s Research Hospital in Memphis, Tennessee.