WASHINGTON — An outbreak of the avian flu virus in the United States could infect one-third of the population and kill as many as 1.9 million people, according to an analysis by the federal government released Wednesday.
The report, by the U.S. Department of Health and Human Services, said that banning large gatherings and imposing quarantines would likely be necessary in order to contain the spread of the H5N1 avian influenza virus if it mutated into a form easily spread by human-to-human contact.
There have been no reported cases of the virus being spread between humans.
The government analysis released Wednesday was part of a plan, broadly outlined by President Bush on Tuesday, to confront threats from new outbreaks of flu. The analysis was based on previous flu pandemics in the United States in 1918, 1957 and 1968.
"When a pandemic virus strain emerges, 25 percent to 30 percent of the population could develop the clinical disease and a substantial fraction of those individuals could die," the report stated. "The resources required for such a vigorous containment would almost certainly exceed those available in the affected communities."
The government analysis found that 40 percent of school-age children could become sick with the virus, and between 865,000 and 9.9 million people would require hospitalizations — depending on its severity — a number that would threaten to overwhelm the hospital system.
The virus would affect a community for six to eight weeks and likely come in two different waves.
The report warned, however, that because such viruses quickly mutate, they are difficult to predict.
"The scope and pace of an influenza pandemic may defy accurate prediction," the report said. "The disease may appear in many different parts of the nation almost simultaneously, or disease may occur in only one or a few communities, and if not contained there, proceed to affect other communities."
Concerns about pandemic flu have intensified in recent years with the spread of a deadly avian flu strain among birds in Asia that has decimated entire flocks. This bird flu has infected about 120 people and killed 60. But the virus has yet to pass easily among humans, as is necessary to create a pandemic. Experts debate whether it ever will, but most believe that a pandemic flu someday is inevitable.
Pandemic flu has struck the nation three times in the past century. The first occasion, and the worst, was in 1918, when 500,000 people in the United States and 20 million worldwide died of what has since been identified as an avian flu. But a pandemic flu could also result from unexpected genetic changes to the seasonal human flus that circle the globe and already cause about 36,000 deaths a year in the United States.