TUESDAY, June 10, 2014 (HealthDay News) -- About 400,000 Americans may have the sexually transmitted disease chlamydia, but not know they have it, new research suggests.

The new government report estimates that 1.8 million people in the United States have chlamydia, but that only 1.4 million infections have been reported.

Women, particularly young women, seem to have an even greater risk of harboring this often symptomless infection, according to the U.S. National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention news release.

Researchers analyzed data from the 2007 to 2012 National Health and Nutrition Examination Survey and found that 1.7 percent of men and women aged 14 to 39 have chlamydia, which works out to about 1.8 million infections in the United States. However, only about 1.4 million chlamydia infections are reported each year, which indicates that many chlamydia infections go undiagnosed, the researchers said.

The chlamydia infection rate is highest among sexually active girls aged 14 to 19, at 6.4 percent, the investigators found. The rate among sexually active boys aged 14 to 19 is 2.4 percent.

The study also found significant racial differences. For example, the rate among sexually active black teen girls is 18.6 percent, compared with 3.2 percent among sexually active white teen girls.

The findings show the importance of screening all sexually active teen girls for chlamydia in order to ensure that all those who are infected get diagnosed and treated, the researchers said.

They added that the racial differences they discovered show the need for targeted interventions, particularly among black teen girls.

The study was scheduled for presentation Tuesday at the STD Prevention Conference in Atlanta.