A new study by the US National Institutes of Health (NIH) has suggested that Covid-19 cases were detected in the country earlier than previously reported.

antibody testing study examining samples originally collected through the National Institutes of Health’s All of Us Research Program found evidence of SARS-CoV-2 infections in five states earlier than had initially been reported. These findings were published in the journal Clinical Infectious Diseases. The results expand on findings from a Centers for Disease Control and Prevention Study that suggested SARS-CoV-2, the virus that causes COVID-19, was present in the U.S. as far back as December 2019.

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NIH researchers analyzed more than 24,000 stored blood samples from all 50 US states between January 2 and March 18, 2020.

They detected antibodies against SARS-CoV-2, the virus that caused Covid-19, using two different serology tests in nine participants’ samples.

Seven of those samples were seropositive prior to the first confirmed case in the states of Illinois, Massachusetts, Wisconsin, Pennsylvania, and Mississippi, according to the study published by the Clinical Infectious Diseases journal.

The study is the latest to suggest the coronavirus first appeared in the US earlier than previously known.

“This study allows us to uncover more information about the beginning of the U.S. epidemic and highlights the real-world value of longitudinal research in understanding dynamics of emerging diseases like COVID-19,” said Josh Denny, M.D., M.S., chief executive officer of All of Us and an author of the study. “Our participants come from diverse communities across the U.S. and give generously of themselves to drive a wide range of biomedical discoveries, which are vital for informing public health strategies and preparedness.”

In studies like these, false positives are a concern, particularly when the prevalence of viral infections is low, as was the case in the early days of the U.S. epidemic. Researchers in this study followed CDC guidance to use sequential testing on two separate platforms to minimize false-positive results.

The US continues to be the worst-hit country with the world’s highest number of cases and deaths at 33,498,145 and 600,648, respectively, according to the Johns Hopkins University.