WASHINGTON — President Donald Trump announced Monday that the federal government will begin distributing millions of rapid coronavirus antigen tests to states this week and urged governors to use them to reopen schools for students in kindergarten through 12th grade.

The move to vastly expand U.S. testing comes as confirmed new COVID-19 cases remain elevated at more than 40,000 per day and experts warn of a likely surge in infections during the colder months ahead. It also comes just five weeks before the November election, with Trump facing continued criticism for his handling of the crisis.

Antigen tests, which can detect a sign of infection before symptoms develop, are not as accurate as lab-based molecular diagnostic tests.. But because these low-cost, compact antigen detection devices provide results within minutes wherever the tests are given, they can potentially be used to screen people in group settings.

The antigen tests will go out to states based on their population and can be used as governors see fit, but the Trump administration is encouraging states to place a priority on schools. White House officials said at a Rose Garden event that 6.5 million tests will go out this week and that a total of 100 million tests will be distributed to governors over the next several weeks.

Officials said the administration is emphasizing antigen testing in schools because getting back into classrooms is important to students’ physical, social and emotional development, as well their academic progress.. The antigen tests, made by Abbott Laboratories, would allow parents to know whether their symptomatic child has COVID-19. In some cases, states could undertake some baseline surveillance, like testing a proportion of students per week or per month to make sure that the incidence of COVID-19 is low.

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“You have too many states that are locked down right now,” Trump said. “The governors are ... nobody knows what the governors are doing actually.”

The tests will come from a previously announced supply of 150 million ordered from Abbott. The company’s rapid test, the size of a credit card, does not require high-tech, lab-based processing, unlike molecular diagnostic tests.. The rapid test delivers results in about 15 minutes.

Rapid, convenient testing is considered essential to reopening the U.S. economy while controlling the virus.. But testing of all sorts has been plagued by problems since the earliest days of the outbreak.

First, the government lost pivotal weeks distributing, then correcting, a flawed molecular diagnostic test developed by U.S. scientists. Then, private labs and hospitals struggled for months to ramp up molecular testing capacity due to shortages of key supplies, including nasal swabs and chemicals. Testing backlogs meant results were delayed to the point of being useless for quarantine and contact tracing.

The issue is politically sensitive for Trump as he grapples with the pandemic that has killed more than 200,000 Americans. For months, Trump has prodded state and local leaders to open schools this fall.

Only in the last two months has U.S. diagnostic testing capacity generally exceeded demand. The government’s top testing official, Adm. Brett Giroir, told Congress last week that the nation will soon have the capacity to run 3 million tests per day, on average. The U.S. has been averaging about 900,000 tests per day, according to data from the COVID Tracking Project.

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Giroir demonstrated the ease with which the test is given, self-administering the nasal swab then placing it on a piece of paper that contained six drops of liquid.

“This is a very sophisticated little piece of cardboard with ... incredible technology,” he said.

Abbott’s test is an important advance because of its low cost and easy-to-use format. It opens the possibility of ongoing surveillance of students, nursing home residents, office workers, and others who have no symptoms.

But rapid, point-of-care tests like Abbott’s have their own downsides. Like molecular tests, antigen tests are a snapshot in time, so a negative result offers only limited reassurance. A positive antigen result usually needs to be followed by a more reliable molecular test. Additionally, because antigen tests are often performed outside the health care system, state officials have warned that many test results are going unreported. That could lead to a misleading picture of the scope of infection and transmission.

“What we’re hearing from the states is that they don’t know where these tests are being done,” said Dr. Jeffrey Engel of the Council of State and Territorial Epidemiologists, in a recent interview. He warned that schools generally do not have the capacity or expertise to report mass testing results, which could artificially lower infection counts sent to state and federal officials.

Trump warned that with an increase in testing, there would “automatically” be an increase in the number of confirmed COVID-19 cases.

“It’s important to remember that as younger and healthier people return to work and as we massively increase testing capacity, we will identify more cases in asymptomatic individuals in low-risk populations,” Trump said. “This should not cause undue alarm.”

The antigen tests from Abbott are being made in two factories, one in Illinois and one in Maine. The company is in a ramping-up phase. The federal government bought the first 150 million, and it will take the rest of the year to completely fill that order. After that, the administration will decide whether the government should purchase more or whether the free market can determine adequate distribution.

The nonprofit Rockefeller Foundation says the U.S. will need roughly 200 million tests per month to safely reopen schools as part of a broader phased approach to easing restrictions, according to a paper issued earlier this month. The report authors noted that the U.S. is currently averaging fewer than 30 million tests per month.

Despite the gap, Rockefeller’s director, Dr. Jonathan Quick, called Monday’s announcement “an exciting and very significant step.” He added that states will need sustained funding and testing supplies “for the foreseeable future.”

The Trump administration announced earlier this month that the Abbott tests would also go to assisted-living facilities, moving to fill a testing gap for older adults who do not need the constant attention of a nursing home. Senior day care centers and home health agencies are getting the tests too.

Long-term care facilities, including nursing homes and assisted living, account for a sliver of the U.S. population but more than 40% of deaths from COVID-19.