THURSDAY, Oct. 29 (HealthDay News) -- Up to 5.7 million cases of H1N1 swine flu hit the United States between April and July, according to a new estimate from the U.S. Centers for Disease Control and Prevention.

That number came from a model that estimated that for every confirmed case of H1N1 flu there were 79 unreported cases, according to a report published Thursday in the CDC journal Emerging Infectious Diseases.

Speaking at an afternoon teleconference on Thursday, Dr. Anne Schuchat, director of CDC's National Center for Immunization and Respiratory Diseases, explained why the CDC uses such models to get an idea of the actual number of H1N1 flu cases.

"Not every case will result in a person seeking medical care," she noted. "Not every person who seeks medical care will be tested for flu. Not every person tested for flu will have a result that is positive. Not every positive result will get reported. So you can imagine that what we count -- whether it's cases or hospitalizations or deaths -- will be underestimates of the full burden."

The report estimates that there were between 1.8 million and 5.7 million cases of flu, as well as 9,000 to 21,000 hospitalizations and about 800 deaths related to the pandemic flu.

There has been no current update of these numbers since July, Schuchat said. "But we do believe many millions of people have already contracted this virus here in the U.S. And by now we have had well over 20,000 hospitalizations," she noted.

Schuchat also cautioned that the 79-to-1 multiplier used in the report is only a best guess and should be taken with "a grain of salt."

She also had some news on the H1N1 vaccine front, noting that the supply of vaccine is changing day by day. As of Thursday there were 24.8 million doses available, Schuchat said, 1.6 million doses more than yesterday.

Production delays have resulted in far fewer doses of the vaccine being available than federal officials had hoped for by this time. The first estimates called for 40 million doses by the end of October and 190 million doses by year's end.

Every day more vaccine is becoming available, however, and officials hope to see an end to the shortage over the next several weeks.

"I appreciate the frustration people are seeing as they are unable to find vaccine," Schuchat said. "Over the next several weeks it should become more easily available and each day we are seeing forward progress."

She also said that supplies of the antiviral drug Tamiflu should be plentiful. While there may be some shortage of the liquid form usually given to children, pharmacists can convert pills into a liquid. Parents too can mix the drug with chocolate syrup to make it more palatable for children who have trouble taking pills, she said.

In related news, Swiss pharmaceuticals giant Novartis AG said on Thursday that it is on schedule to meet its U.S. government contract for H1N1 vaccine, the Associated Press reported.

Fears had circulated in recent weeks that the company might have to delay shipments of the sought-after vaccine, and U.S. officials had announced earlier in October that they had only received 23 million of a predicted 45 million doses by mid-month. However, Novartis remains on target to deliver 25 to 30 million doses of the vaccine to the United States by the end of November, the AP said.

More information

For more information on H1N1 swine flu, visit Flu.gov.

SOURCES: Oct. 29, 2009, teleconference with: Anne Schuchat, M.D., director, National Center for Immunization and Respiratory Diseases, U.S. Centers for Disease Control and Prevention; Oct. 29, 2009, CDC, Emerging Infectious Diseases; Associated Press

Copyright © 2009 ScoutNews, LLC. All rights reserved.