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The CDC on Monday, raising concerns from public health experts who have repeatedly said that widespread testing is key to getting the pandemic under control.

Previously, the CDC recommended testing for all close contacts of people infected with SARS-CoV-2, even if they didn't have symptoms. Now, close contacts (within 6 feet, more than 15 minutes) "do not necessarily need a test unless you are a vulnerable individual" or if state and local health authorities recommend it.

Although calls for close contacts to isolate at home for 14 days after exposure, the new statement on testing merely calls for those individuals to "strictly adhere to CDC mitigation protocols" without further explanation.

The move has left public health experts baffled, as the consensus is that a substantial proportion of transmission is driven by asymptomatic spread.

"Close to 40% of infections are asymptomatic, and asymptomatic people transmit infection," Carlos del Rio, MD, of Emory University in Atlanta, told CNN. "If you've been in contact with someone for a few minutes, that's okay, but if you've been in contact with someone for 15 minutes, without a mask, I think you need to be tested regardless of whether you have symptoms or not."

The CDC did not offer any evidence for the change to the guidelines. When asked to produce that evidence by 麻豆传媒, a CDC press person directed the request to the HHS Office of the Assistant Secretary for Health. That office did not return a request for comment as of press time.

"This is the kind of policy change where it would be really helpful to hear CDC talk through their thinking in a press conference. I wish we could hear from them more," , of Johns Hopkins Center for Health Security in Baltimore.

Leana Wen, MD, of George Washington University, : "I wish we had a briefing led @CDCgov every day that leads with data, based on science."

During an afternoon press briefing with reporters, Assistant Secretary for Health Adm. Brett Giroir, MD, said the guidelines "are a CDC action" that received input from all members of the coronavirus task force.

But an unnamed government official "close to the process" that the CDC was pressured "from the top down" to change the testing guidance.

Giroir said the guidance was intended to make testing more strategic, focusing asymptomatic testing on high-prevalence areas, and putting more responsibility on local public health officials to decide which patients need testing.

"We want appropriate testing, not less testing," Giroir said. "We want more asymptomatic testing in areas where it's needed and hopefully less where it's not needed."

In an emailed statement on Thursday morning, CDC Director Robert Redfield, MD, echoed Giroir's comments, stating that "Everyone who wants a test does not necessarily need a test; the key is to engage the needed public health community in the decision with the appropriate follow-up action."

Redfield's statement noted that testing "may be considered for all close contacts of confirmed or probable COVID-19 patients." Some in the consumer media interpreted that as the guidance update, even though the latter had specifically allowed testing when recommended by a healthcare professional.

Neither Giroir nor Redfield, however, filled in the data gap lamented by public health advocates.

In the absence of data, experts have speculated as to the reasoning behind the change.

"One has to wonder: is this change because we don't have enough tests?" Supply shortages have plagued the U.S. pandemic response from the beginning, and have not appeared to improve over the last several months.

Yet about diagnostic tests' high rates of false negatives, particularly early on in infection, which could be problematic if incorrect results give patients a false sense of security.

Giroir cited false negatives as a major reason for the new guidance. Given that the median time to developing symptoms after exposure is about 7.5 days, he said, getting a negative test "at 3 days [is] not totally meaningless, but close to it. It should not give you a false sense of security."

"A negative tegative test on day 2 doesn't mean you're negative," Giroir said. "It doesn't mean you can go out on day 4 to visit grandma or that on day 6 you're going out without wearing a mask."

But Giroir provided no actual data on positivity rates in asymptomatic close contacts of infected individuals, nor on false negative rates in this population.

Adam Gaffney, MD, MPH, of Harvard Medical School, told 麻豆传媒 that while there are legitimate concerns about a high false-negative rate early after exposure, "in that case, we should be talking about guidance about the appropriate testing interval after exposure -- not abandoning testing of close contacts altogether."

Experts also point to political motivations for the decision. "Over and over, the president has said, 'We have a lot of cases because we do a lot of testing.' If we slow down the testing, we will have a decrease in cases," del Rio told CNN. "I am worried this is just a way to slow down testing, and that would not be good. We don't want to decrease testing, we want to decrease cases by decreasing transmission."

Earlier this week, public health experts were alarmed by the FDA's overstatement of a treatment benefit for convalescent plasma during a press briefing announcing the agency's emergency use authorization for the therapy. Similar questions about politics playing too large a role in that decision were also raised.

"This is playing out in the context of a hyper-politicized moment," Gaffney told 麻豆传媒 at the time. "It behooves everyone to be extremely cautious in speaking about these things to avoid the appearance of politicization."

Celine Gounder, MD, an infectious disease expert at New York University, said on CNN that "this week the CDC and FDA suffered real blows to their credibility as scientists and public health agencies who are supposed to advocate for what's in the best interest of the public."

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