The country had roughly 56,300 new HIV infections in 2006 -- a dramatic increase from the 40,000 annual estimate used for the past dozen years. The new figure is because of a better blood test and new statistical methods, not a worsening of the epidemic, officials said.
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, the 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 today, 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.
David Holtgrave, of Johns Hopkins University. 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.
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 might 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 estimate is based on a blood test that for the first time can tell how recently an HIV infection occurred, distinguishing infections that occurred within the past five months from those that were older.
Past tests could detect only the presence of HIV, so determining which year an infection took place was guesswork -- guesswork the old 40,000 estimate was based on.
The new estimate relies on blood tests from 22 states.
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 infections are falling among heterosexuals and injection drug users.