DENVER -- In a disturbing new projection, health officials say up to 40 percent of Americans could get the H1N1 virus this year and next and several hundred thousand could die without a successful vaccine campaign and other measures.

The estimates by the Centers for Disease Control and Prevention are roughly twice the number of those who catch flu in a normal season and add greater weight to hurried efforts to get a new vaccine ready for the fall flu season.

H1N1 has already hit the United States harder than any other nation, but it has struck something of a glancing blow that's more surprising than devastating. The virus has killed about 300 Americans and experts believe it has sickened more than 1 million, comparable to a seasonal flu with the weird ability to keep spreading in the summer.

Health officials said flu cases may explode in the fall, when schools open and become germ factories, and the new estimates dramatize the need to have vaccines and other measures in place.

But because the H1N1 mortality rate has, up to now, been minimal, some are dismissing the severe warnings.

Daniel Williams, a Denver resident, was diagnosed with H1N1 two weeks ago.

"I was under the impression that it was going to be a very turbulent recovery," he said.

But it was no worse than a minor bout of seasonal flu. Williams said by the time he tested positive for the virus, his symptoms were all but gone.

"I've had colds that were far worse than this," he said.

Dr. Connie Price, chief of infectious disease at Denver Health Medical Center, said so far, data shows H1N1 is no more potent than the seasonal flu virus, although, unlike season flu, it does have the bizarre ability to continue to spread during the summer months and seems to affect young, healthy people more seriously.

Price said doctors are waiting on additional data and can only speculate about how serious H1N1 could get.

"We've only had three of these episodes in our history in which to base all of these conclusions on," said Price. "We are guessing."

Price said health officials are tracking the progression of H1N1 in the southern hemisphere and more data should be available this fall.

A world health official said the first vaccines are expected in September and October. The United States expects to begin testing on some volunteers in August, with 160 million doses ready in October.

Price said basic sanitation could severely limit the likelihood of getting the virus.

"It sounds simple, but it is effective: hand-washing, cover your cough, stay home when you're sick," she said.