Up to 5.7 mln US swine flu cases in four months: study

Up to 5.7 million people in the United States may have been infected with swine flu in the first four months of the outbreak, or more than 100 times the number of laboratory-confirmed cases that were reported, a study said Thursday.

Using a multiplier model, researchers from the Centers for Disease Control and Prevention demonstrated that "the reported cases of laboratory confirmed pandemic (H1N1) 2009 are likely a substantial underestimation of the total number of actual illnesses that occurred in the community during the spring of 2009," lead author Carrie Reed wrote in the study.

The researchers estimated that between 1.8 million to 5.7 million cases of H1N1 flu occurred in the United States in the four months from April, when the virus was first reported.

Of those cases, between 9,000 and 21,000 were hospitalized, the report estimated.

Although the report did not estimate the number of deaths from swine flu, Read pointed out that the ratio of deaths to hospitalizations in the four months to July 23 was six percent.

That would mean that around 1,200 people died of H1N1 flu in the United States in the first months of the outbreak.

The official US death rate from swine flu at the end of July was less than 300.

"We have been saying that we were just finding the tip of the iceberg with our laboratory confirmed reporting," Anne Schuchat, the director of the national center for immunization and respiratory diseases, told reporters on Thursday as she commented on the report.

The danger from the hugely underestimated impact of swine flu is that health authorities and infrastructures might be "unprepared in the short-term" to tackle the H1N1 outbreak.

US health officials have said vaccine supply would fall about 10 million doses short of the 40 million doses they had expected to have by the end of October, and long lines have formed outside vaccination clinics around the United States, with many people turned away as supplies of vaccine ran dry. (AFP)