Federal health officials now say 4,000 or more Americans likely have died from swine flu - about four times the estimate originally reported.
The new, higher figure includes deaths caused by complications related to swine flu, such as pneumonia and bacterial infections.
The Centers for Disease Control and Prevention (CDC) had previously put the U.S. swine flu death count at more than 1,000.
The new numbers come at a time when a vaccine for the virus is in short supply.
"Unfortunately, the consequences of not having enough vaccine soon enough is we're going to have people who are going to necessarily get sick and some will die," the University of Minnesota's Dr. Michael Osterholm said.
Margaret Savitts' husband came down with what seemed to be the regular flu - it turned out to be the swine flu.
"By Saturday night he couldn't breathe," she said. "He was having a really hard time. And by 2 a.m. Monday morning he was in full respiratory failure."
Sanofi-aventis, the only company making the vaccine in the U.S., is working around the clock to erase a backlog in vaccine production.
"For us there is no delay," said Sanofi-aventis CEO Chris Veinbacher. "It has always been a period of time that is very fixed. We're talking about Mother Nature here."
The company hopes to deliver 75 million doses of the vaccine before the end of the year.
Public health officials say the revised death toll is well below the yearly toll from the seasonal flu.
Meanwhile, thousands across the nation have been lining up for the swine flu vaccine.