COVID-19 infections were present in the United States as early as mid-December 2019, weeks before it was first identified in China and about a month earlier than the first case was officially confirmed in the United States, according to a new study by the U.S. Centers for Disease Control and Prevention (CDC).

In the study published in the journal Clinical Infectious Diseases, CDC researchers tested blood samples from 7,389 routine blood donations collected by the American Red Cross from Dec. 13, 2019 to Jan. 17, 2020 for antibodies specific to the novel coronavirus, known as SARS-CoV-2.

The study aims to determine if SARS-CoV-2 reactive antibodies were present in sera prior to the first identified case in the United States on Jan. 19 this year.

The researchers found evidence of infection in 106 of 7,389 blood donations from residents in nine states across the United States.

Antibodies were found in 39 samples from California, Oregon and Washington state collected between Dec. 13 and Dec. 16, and 67 samples in Massachusetts, Michigan, Wisconsin or Iowa, and Connecticut or Rhode Island collected between Dec. 30 and Jan. 17.

The findings suggest that SARS-CoV-2 infections may have been present in the United States in December 2019, earlier than previously recognized, the authors wrote in the study.

The study also highlighted the value of screening routinely collected blood samples for evidence of viruses spreading in a population, said the researchers, adding the CDC is continuing to conduct ongoing surveillance using blood donations and clinical laboratory samples for SARS-CoV-2 infection in multiple sites across the country.

The results add to growing evidence that COVID-19 was circulating outside of China earlier than previously known.

The first COVID-19 infection in the United States was reported on Jan. 19, 2020 in a returned traveler from China, two days after domestic testing was initiated, according to the CDC.

Two other people who were subsequently diagnosed with COVID-19 in the United States also developed symptoms in mid-January.

Some reports have suggested the introduction of SARS-CoV-2 into the United States may have occurred earlier than initially recognized, though widespread community transmission was not likely until late February, according to the study.