Coronavirus infections could be up to 13 times higher across US than initially reported, CDC finds

Antibody test results from 10 cities suggest Americans were spreading the virus without realising they were infected

Coronavirus infections across the US are likely much higher than initially reported, according to a newly released report from the Centres for Disease Control and Prevention.

The agency's report suggests that people who did not present any Covid-19 symptoms unknowingly transmitted the virus in their communities, underscoring health officials' early warnings that testing is only capturing a fraction of the scale of infections.

For most areas, "it is likely that greater than 10 times more" infections occurred than there were cases reported, though most residents were not symptomatic, the report says. With nearly 4 million cases identified in the US since the onset of the outbreak, the CDC's report suggests more than 40 million people were infected.

Data from antibody tests performed in 10 cities found that reported infections "likely underestimate the prevalence of infection in affected communities," according to the CDC. Antibody tests can be used to determine prior infections.

"The findings may reflect the number of persons who had mild or no illness or who did not seek medical care or undergo testing but who still may have contributed to ongoing virus transmission in the population," the agency said.

In Missouri, the CDC discovered that the infection rate is 13 times higher than the reported rate, The New York Times reported. Utah's infections were twice as high as reported.

The CDC collected blood samples taken during routine screenings from late March through mid-April, capturing the state of the virus before a recent surge in infections that began in June.

A report released in June showed data from just six cities. The July study published by the Journal of the American Medical Association includes four others.

Blood samples were also collected from commercial labs in Connecticut, Louisiana, Minnesota, New York City, Philadelphia, the San Francisco Bay Area, south Florida, Utah and Washington state.

New York's infection rate increased from 6.9 per cent on 1 April to more than 23 per cent by 6 May, mirroring the scale of the outbreak as the state became a global epicentre for the virus.

But the results show that even in highly impacted areas like New York City, the rate of infections is still far lower than needed to meet the 60 to 70 per cent rate threshold for "herd immunity".

Roughly 40 per cent of people who are infected do not present any symptoms. Health officials and medical experts have stressed that Americans should follow preventative guidelines like using face coverings and remaining physically distant from others in public to prevent unknowingly transmitting ot exposing oneself to the virus.

More than 3.8 million confirmed infections have been reported in the US, according to Johns Hopkins University. The US currently tests roughly 700,000 people daily. The nation's death toll has surpassed 140,000.