CHICAGO (Reuters) - As many as 5.7 million Americans were infected with the H1N1 virus between April and late July, U.S. researchers said on Thursday, offering the clearest picture yet of how quickly and widely swine flu can spread.

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Researchers used computer models to estimate the number of people who have contracted swine flu, which began infecting Americans in April.

They estimated that 1.8 million to 5.7 million cases of swine flu occurred between April and July 23, sending between 9,000 and 20,000 people to the hospital.

About 6 percent of people who were hospitalized with the virus died, the team, led by Carrie Reed at the U.S. Centers for Disease Control and Prevention, reported in the journal Emerging Infectious Diseases.

This suggests that as many as 1,300 people died from their infections between April and July. Officially, 1,000 U.S. deaths have been attributed to H1N1 since April.

Dr. Anne Schuchat of the CDC said on Thursday the agency does not have an update beyond July 24.

“We do believe many millions of people have already contracted this virus in the United States,” Schuchat said.

“It’s probably now well more than 20,000 hospitalizations,” she said. “Really, the priority is to minimize illness and death.”

Part of the U.S. plan to do that was through widespread vaccinations, but manufacturing delays have stalled those efforts. “We had all hoped to have more vaccine now than we have,” Schuchat said.

Earlier government estimates had suggested there would be as many as 40 million vaccine doses available for state and local health authorities to distribute by the end of October.

Schuchat said 24.8 million doses of the H1N1 vaccine are available, 1.6 million more doses than on Wednesday.

The United States has ordered up to 250 million doses of H1N1 vaccine from five companies -- MedImmune, a unit of AstraZeneca, Sanofi-Aventis, Australia’s CSL, GlaxoSmithKline and Novartis.

Except for MedImmune, all had problems making vaccine at first and are still struggling to make the virus grow in eggs, the first step to manufacturing influenza vaccine.

Schuchat said state and local health departments have had to adapt their vaccination plans to cope with the delays, and dole out a limited number of doses to people at greatest risk of developing severe disease from H1N1, including people with underlying health conditions and women who are pregnant.

Several studies released at the meeting of the Infectious Diseases Society of America in Philadelphiaon Thursday showed that vaccinating pregnant women protected their babies, also.

They said babies were less likely to be premature and were bigger if their mothers were vaccinated against flu. A separate study showed that people who had been taking cholesterol lowering drugs called statins were less likely to die from flu.