Washington, Jun 25 (IANS): Nearly 17 million Americans went undiagnosed for Covid-19 in the US by mid-July 2020, according to a new study by the National Institutes of Health.

The study suggests that the prevalence of Covid-19 in the US during spring and summer of 2020 far exceeded the known number of cases and that infection affected the country unevenly.

For every diagnosed Covid-19 case during that time, the researchers estimate that there were 4.8 undiagnosed cases, representing an additional 16.8 million cases by July alone.

"This study helps account for how quickly the virus spread to all corners of the country and the globe," said Bruce Tromberg, Director of the National Institute of Biomedical Imaging and Bioengineering (NIBIB), one of the NIH institutes who run the NIH SARS-CoV-2 Seroprevalence Project.

"This wide gap between the known cases at the time and these asymptomatic infections has implications not only for retrospectively understanding this pandemic, but future pandemic preparedness," said senior co-author Kaitlyn Sadtler, chief of the NIBIB Section on Immunoengineering.

The finding was published in the early online issue of Science Translational Medicine and represents the first data from the 12-month NIH study that was launched in April 2020.

The team recruited over 240,000 volunteers from across the country, then selected 8,058 individuals from that pool, who were not previously diagnosed with SARS-CoV-2 infection and analysed their blood samples.

They found that 304 of the approximately 8,000 blood samples were seropositive, meaning that they contained antibodies against the SARS-CoV-2 virus.

The researchers estimated that 4.6 per cent of the US adults had undiagnosed Covid-19 during their study period. The youngest participants -- those between the ages of 18 and 44 -- had the highest estimated seropositivity, at 5.9 per cent.

The estimated seropositivity was higher in females than in males (5.5 per cent versus 3.5 per cent, respectively). And urban participants had a higher estimated seropositivity (5.3 per cent) compared with rural participants (1.1 per cent).

"The estimate of Covid-19 cases in the United States in mid-July 2020, 3 million in a population of 330 million, should be revised upwards by almost 20 million when the percent of asymptomatic positive results is included," Sadtler said.