WASHINGTON — U.S. officials estimate that 20 million Americans have been infected with the coronavirus since it first arrived in the United States, meaning that the vast majority of the population remains susceptible.

Cars are lined up at a United Memorial Medical Center COVID-19 testing site Wednesday in Houston. Texas Gov. Greg Abbott said Wednesday that the state is facing a “massive outbreak” in the coronavirus pandemic and that some new local restrictions may be needed to protect hospital space for new patients. David J. Phillip/Associated Press

Thursday’s estimate is roughly 10 times as many infections as the 2.3 million cases that have been confirmed. 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 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.

Twenty million infections means that about 6 percent of the nation’s 331 million people have been infected.

“It’s clear that many individuals in this nation are still susceptible,” Dr. Robert Redfield, director of the Centers for Disease Control and Prevention, said on a call with reporters Thursday. “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 percent of infected people might not have symptoms.

“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.”

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 prioritized testing for those with symptoms.

Administration officials are pointing to the new data to allay public anxieties, claiming that while there have been significant spikes, they have the outbreaks well in hand. They maintained they were not trying to minimize a public health crisis, but said the nation is in a markedly different place with the virus now than when the U.S. last saw similar infection numbers in mid-April, when testing infrastructure was weaker. The officials spoke on condition of anonymity late Wednesday to discuss the matter candidly.

“This is still serious,” Redfield said on the call with reporters Thursday. “But I’m asking people to recognize that we’re in a different situation today than we were in March or April,” with more cases today in younger people who are not as likely to develop serious illness or die from infection, he said.

President 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 hospitalizations in some areas.

Redfield said infection prevalence rates ranged from 1 percent in some areas of the West Coast to much higher in New York City.

Several independent experts said the methods and locations of sampling are key to interpreting the numbers.

A healthcare professional walks through the Texas Medical Center on Thursday in Houston. The leaders of several Houston hospitals said they were opening new beds to accommodate an expected influx of patients with COVID-19, as coronavirus cases surge in the city and across the South. David J. Phillip/Associated Press

Dr. Thomas Tsai, a Harvard University health policy researcher, said 20 million seems reasonable, but “most of these estimates exist in a range” and it’s important to know how wide that is.

“It’s hard to interpret this just from a single number and without the context for it,” such as what locations were sampled and whether it was truly a random slice of a population or areas of low or high prevalence, which can skew the results.

Justin Lessler, a researcher at Johns Hopkins Bloomberg School of Public Health, said 20 million infected “is about what you’d expect based on the case fatality rate” and the number of deaths that have occurred so far in the U.S.

Despite the phaseout of daily White House coronavirus briefings, the administration has been closely monitoring data on the spread of COVID-19 and has been deploying teams from the CDC to identify and stem outbreaks around the country.

A dozen states in recent weeks have seen a worrisome uptick in new cases, as well as in the more critical measure of the percentage of positive cases discovered in tests performed. Seven states have seen more than 10 percent of tests come back positive. And troublesome spikes in Sun Belt states have dominated news coverage in recent days, to the consternation of Trump administration officials.

Coronavirus cases and deaths The Washington Post

They point instead to more nuanced county-level data, which shows positivity rates exceeding 10 percent in just 3 percent of the nation’s counties. Yet they acknowledge that some of the areas with the highest transmission rates are generally the most populous, suggesting tens of millions of Americans could be living in areas with spiking infections.

As states reopen, the administration says it is up to governors and local officials to determine how to respond to the spikes. Texas GOP Gov. Greg Abbott, for example, put a hold on any further steps to reopen and reimposed a ban on elective surgeries in some areas to preserve hospital space after the number of patients statewide more than doubled in two weeks.

The CDC teams, officials said, are working to trace new outbreaks and reinforce protective measures like social distancing and mask wearing in hard-hit areas and to remind vulnerable populations to take extreme precautions. The administration says those efforts have helped slow new infections in North Carolina and Alabama, where they were deployed earlier this month.

One of the hard-hit areas is Phoenix, where Trump held an event Tuesday with thousands of young attendees, nearly all of whom were maskless.

The officials say the nature of the outbreak now is different than months ago, when deaths topped more than 1,000 per day for weeks and hospitals were stretched beyond capacity across the country. The new increase in positive cases, they said, is capturing what has long been there. They say it is only now showing up in data because the U.S. has increased testing and surveillance.

Meanwhile, mortality data has steadily declined, as have hospitalizations in all but a few hot spots.

To the administration officials, that reinforces their hypothesis that millions unknowingly had the virus earlier this year. Through early May, federal guidelines prioritized testing for symptomatic people, those exposed to a positive case and those in high-risk environments.

With testing far more widespread now, officials believe 50 percent of new cases in Florida and Texas are among people ages 35 or younger, and most of them are asymptomatic.

The U.S. is testing about 500,000 patients per day. On a per capita basis, the U.S. rate falls behind several other countries, including Spain, Australia, Russia and Iceland, according to figures from Johns Hopkins University.

There is no scientific consensus on the rate of testing needed to control the virus. But many experts say the U.S. should be testing roughly 1 million to 3 million people daily to catch new cases and prevent flareups. Administration officials said they expect to be able to run 20-40 million tests per month beginning this fall.

Testing is constrained by laboratory supplies needed to run the tests but also lack of demand for tests in some areas.

Marchione reported from Milwaukee. Associated Press writer Matthew Perrone in Washington contributed to this report.