Officials estimate that 20 million Americans have been infected with the coronavirus since it first arrived in the United States.
The figure, from the Centres for Disease Control and Prevention (CDC), is roughly 10 times the number of confirmed cases which is 2.3 million.
Officials have long known that millions of people were infected without knowing it and that many cases are being missed because of gaps in testing.
The news comes as the Trump administration works to play down nationwide concern about the Covid-19 pandemic as about a dozen states are seeing increases in cases.
If accurate, the 20 million figure would mean that about 6% of the US population have been infected.
Dr Robert Redfield, director of the Centres for Disease Control and Prevention, said: “It’s clear that many individuals in this nation are still susceptible,
“Our best estimate right now is that for every case that was reported, there actually are 10 more infections.”
Previously, CDC officials and the nation’s top infectious disease expert, Dr Anthony Fauci, have said that as many as 25% of infected people might not have symptoms.
The new estimate is based on CDC studies of blood samples collected nationwide – some by the CDC and others from blood donations and other sources.
Many infections were not caught in early testing, when supplies were limited and federal officials prioritised testing for those with symptoms.
US president Donald Trump, who refuses to wear a face mask in public, has tried to play down the risk.
He told a crowd in Wisconsin on Thursday that the administration had done an “incredible job” fighting the virus and added that “if we didn’t test, we wouldn’t have cases,” which ignores other indicators of the extent of the problem such as surging hospital admissions in some areas.