But it likely will refocus U.S. attention from the effect of AIDS overseas to what the disease is doing to this country, said public health researchers and officials.
"This is the biggest news for public health and HIV/AIDS that we've had in a while," said Julie Scofield, executive director of the National Alliance of State and Territorial AIDS Directors.
Experts in the field, advocates and a former surgeon general called for more aggressive testing and other prevention efforts, noting that spending on preventing HIV has been flat for seven years.
The revised estimate by the Centers for Disease Control and Prevention and the methodology behind it were to be presented Sunday, the opening day of the international AIDS conference in Mexico City.
Since AIDS surfaced in 1981, health officials have struggled to estimate how many people are infected each year. It can take a decade or more for an infection to cause symptoms and illness.
One expert likened the new estimate to adding a good speedometer to a car. Scientists had a good general idea of where the epidemic was going; this provides a better understanding of how fast it's moving right now.
"This puts a key part of the dashboard in place," said the expert, David Holtgrave of Johns Hopkins University.
Judging by the new calculations, officials believe annual HIV infections have been hovering around 55,000 for several years.
"This is the most reliable estimate we've had since the beginning of the epidemic," said Dr. Julie Gerberding, the CDC's director. She said other countries may adopt the agency's methodology.
According to current estimates, around 1.1 million Americans are living with the AIDS virus. Officials plan to update that number with the new calculations but don't think it will change dramatically, a CDC spokeswoman said.
The new infection estimate is based on a blood test that for the first time can tell how recently an HIV infection occurred.
Past tests could detect only the presence of HIV, so determining which year an infection took place was guesswork -- guesswork upon which the old 40,000 estimate was based.
The new estimate relies on blood tests from 22 states where health officials have been using a new HIV testing method that can distinguish infections that occurred within the past five months from those that were older.
The improved science will allow more real-time monitoring of HIV infections. Now, CDC officials say, the estimate will likely be updated every year.
Yearly estimates allow better recognition of trends in the U.S. epidemic. For example, the new report found that infections are falling among heterosexuals and injection drug users.
Some experts celebrated that finding, saying it's a tribute to prevention efforts, including nearly 200 syringe exchange programs now operating in 36 states despite a federal ban on funding for such projects.
But they also lamented the CDC's finding that infections continue to increase in gay and bisexual men, who accounted for more than half of HIV infections in 2006. Also, more than a third of those with HIV are younger than 30.
Some advocates say that suggests a need for more prevention efforts, particularly targeting younger gay and bisexual men.
For years, AIDS was considered a terrifying death sentence, and since 1981, more than half a million Americans have died. But medicines that became available in the 1990s turned it into a manageable chronic condition for many Americans, and attention shifted to Africa and other parts of the world.
Last week, President Bush signed a $48 billion global AIDS bill to continue a program that he called "the largest commitment by any nation to combat a single disease in human history."
But some advocates complain that CDC's annual spending on HIV prevention in the United States has been held to roughly $700 million since 2001, while costs have risen. (That's about 3 percent of what the federal government spends on AIDS; much of the rest is on medicines, health care and research.)
The new estimate is "evidence of a failure by government and society to do what it takes to control the epidemic," said Julie Davids, executive director of the Community HIV/AIDS Mobilization Project.
Whether more funding comes or not, the revised estimate clearly is a "wake-up call to scale things up," said Dr. Kevin Fenton, who oversees CDC's prevention efforts for HIV/AIDS.
Some said more attention needs to focus on prevention among blacks, who account for nearly half of annual HIV infections, according to the new CDC report.
A recent report by the Black AIDS Institute concluded that if black Americans were their own nation, they would rank 16th in the world in the number of people living with HIV.
"We have been inadequately funding this epidemic all along. We need to step it up," said former U.S. Surgeon General Dr. David Satcher, who is now an administrator at Atlanta's Morehouse School of Medicine.
The new estimate has been anticipated for a long time. The CDC began working on the new methods nearly seven years ago.
Late last year, advocates said they had heard the figure was about 55,000 and pressed the CDC to release it. Agency officials declined, saying they were submitting their research for medical journal review.
"These are extremely complicated statistical methods," and CDC officials wanted the work to be thoroughly reviewed by outside experts, Gerberding said. The CDC's findings are being published in the Journal of the American Medical Association.
Until 1992, the number of diagnosed AIDS cases was used to predict how many people were newly infected each year. That method produced an estimate of 40,000 to 80,000. More recently, the CDC focused on infections among men who have sex with men, who account for about half of new HIV diagnoses.