An abrupt shift this week in government testing guidelines for Americans exposed to the novel coronavirus was directed by the White House coronavirus task force, surprising and dismaying many public health experts.

The new guidance, introduced this week without any announcement in a posting on the website of the Centers for Disease Control and Prevention, eliminated advice that everyone exposed to the virus through close contact with an infected individual get tested to find out whether they are positive, regardless of whether they have symptoms.

Several leading infectious-disease experts say they feared the change will increase public confusion and further spread of the disease. The CDC estimates that 40 percent of those infected with the coronavirus have no symptoms but may spread it to other people.

Brett Giroir, an assistant Health and Human Services secretary who coordinates coronavirus testing, said the revision does not reflect any effort to reduce testing for the potentially lethal virus. He said it reflects evolving understanding that a negative test result a few days after exposure may give someone false confidence that they have not become infected.

“A negative test on day two (after being exposed) doesn’t mean you are negative, so what is the value of this?,” Giroir said on a conference call with reporters. “It doesn’t mean on day four, you can go visit grandma or day 6 you can go out w/out a mask.”

The change is CDC guidance, but it was authorized by the coronavirus task force, which approved it last Thursday after about a month of debate, Giroir said.

He said that the idea had originated with himself and CDC director Robert Redfield. He said all the physicians on the task force reached consensus, including Anthony S. Fauci, director of the National Institutes of Health’s National Institute of Allergy and Infectious Diseases. Asked for comment, the CDC referred all questions to HHS, and Fauci could not be reached. The White House offered no immediate comment.

One recent addition to the White House task force, Scott Atlas, a physician and fellow at Stanford University’s conservative Hoover Institution, has spoken out publicly about his view that fewer people need tests for the virus, which has led to more than 5.7 million cases in the United States and at least 175,000 deaths. Atlas is not an infectious-disease specialist.

In its new form, the testing guidance says that, for people who have been within six feet of an infected person for at least 15 minutes, “you do not necessarily need a test.” The previous federal guidelines had urged tests for people who had been exposed, whether they had developed symptoms of not.

The new iteration says exposed people without symptoms still might warrant a test if they are especially vulnerable to the virus or if one is recommended by their source of medical care or by state or local public health officials.

In a statement, Giroir said the update is intended to “reflect current evidence and best public health practices.” He said the current recommendation emphasizes tests for individuals with symptoms consistent with COVID-19, and others without symptoms who are at high risk, including nursing home residents, health care workers, first responders and people who have been identified as susceptible to severe illness or death if they become infected.

Former CDC Director Tom Frieden said that reducing testing among individuals exposed to an infected person could be detrimental.

“Not testing asymptomatic contacts may allow the spread of disease,” he said. There’s a big difference between not testing asymptomatic college students and not testing contacts of an exposed person.

Frieden said that, because testing materials and labs’ capacity have been stretched thin, it makes sense to set priorities for who needed to get tested the most. “But that’s not what they’re saying,” he said. “They’re saying don’t test asymptomatic people.”

He noted that people who are asymptomatic are able to spread the virus to others before they develop symptoms. “(W)e don’t know what proportion of all spread comes from people who are asymptomatic,” Frieden said. “We know it’s not negligible.”

The new version of the guidance also says that someone who has been in a place with high COVID-19 transmission and has attended a public or private gathering of more than 10 people without widespread mask-wearing or physical distancing does “not necessarily need a test” unless that person is a vulnerable individual, or the person’s health-care provider or state or local public health departments recommend a test.

Some public health experts suggested the Trump administration’s advice might stem from a recent increase in the turnaround times for test results in parts of the country. But a senior administration official, speaking on condition of anonymity about data not yet released, said those times have fallen again, before the new guidelines were issued.