"Our best estimate right now is that for every case that was reported, there actually are 10 more infections," Dr Robert Redfield, director of Centers for Disease Control and Prevention says.
Government experts believe more than 20 million Americans could have contracted the coronavirus, 10 times more than official counts, indicating many people without symptoms have or have had the disease, senior US administration officials have said.
"It's clear that many individuals in this nation are still susceptible," Dr Robert Redfield, director of the Centers for Disease Control and Prevention (CDC), said on a call with reporters on Thursday.
"Our best estimate right now is that for every case that was reported, there actually are 10 more infections."
Cases rose across the United States by at least 39,818 on Thursday, the largest one-day increase of the pandemic taking infection cases to 2.5 million.
Six percent of population infected
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.
Twenty million infections means that about six percent of the nation's 331 million people have been infected.
"There's an enormous number of people that are still vulnerable," said Dr Joshua Sharfstein, vice dean at the Johns Hopkins Bloomberg School of Public Health.
"It still remains a potentially lethal disease. It's a roll of the dice for everybody who gets the illness. Also, you're rolling the dice for other people who you may give the virus to."
Infections surge in 29 states
Twenty-nine states are now experiencing new surges, with new cases recorded on Thursday, according to CDC — a level approaching a new record — with 692 new deaths reported.
In a likely sign of the situation's seriousness, Vice President Mike Pence announced that the White House coronavirus task force would give a press conference Friday at the Department of Health and Human Services, the first of its kind in almost two months.
The news comes as the Trump administration works to tamp down nationwide concern about the Covid-19 pandemic as about a dozen states are seeing worrisome increases in cases.
The administration also looks to get its scientific experts back before the public more as it tries to allay anxieties about the pandemic while states begin reopening.
Since mid-May, when the government began stressing the need to get the economy moving again, the panel's public health experts have been far less visible than in the pandemic's early weeks.
Texas stops reopening
The governor of Texas temporarily halted the state's reopening on Thursday as Covid-19 infections and hospitalisations surged.
Texas, which has been at the forefront of efforts to reopen devastated economies shut down in the face of the coronavirus pandemic, has seen one of the biggest jumps in new cases, reporting more than 6,000 in a single day on Monday.
"The State of Texas will pause any further phases to open Texas as the state responds to the recent increase in positive Covid-19 cases and hospitalisations," Governor Greg Abbott's office announced in a statement.