(CNN) In a shift that perplexed some doctors, the US Centers for Disease Control and Prevention has changed its Covid-19 testing guidelines to say some people without symptoms may not need to be tested, even if they've been in close contact with someone known to have the virus.
Previously, the CDC said viral testing was appropriate for people with recent or suspected exposure, even if they were asymptomatic.
Dr. Leana Wen, an emergency physician and public health professor at George Washington University who was previously Baltimore's health commissioner, said on CNN's New Day on Wednesday that the testing guideline changes make no sense.
"These are exactly the people who should be tested," Wen said, giving the example of a person exposed at work who wants a test so they can protect their family at home.
A senior federal health official close to the process tells CNN the sudden change in CDC Covid-19 testing guidance was the result of pressure from the Trump administration. When asked by CNN whether the CDC was responding to pressure from the White House and the Department of Health and Human Services, the senior official said, "It's coming from the top down."
Here's what the CDC website said previously: "Testing is recommended for all close contacts of persons with SARS-CoV-2 infection. Because of the potential for asymptomatic and pre-symptomatic transmission, it is important that contacts of individuals with SARS-CoV-2 infection be quickly identified and tested."
The CDC changed the site on Monday. Here's what it says now: "If you have been in close contact (within 6 feet) of a person with a COVID-19 infection for at least 15 minutes but do not have symptoms, you do not necessarily need a test unless you are a vulnerable individual or your health care provider or State or local public health officials recommend you take one."
Those who don't have Covid-19 symptoms and haven't been in close contact with someone with a known infection do not need a test, the updated guidelines say.
"Not everyone needs to be tested," the agency's website says. "If you do get tested, you should self-quarantine/isolate at home pending test results and follow the advice of your health care provider or a public health professional."
The CDC guidelines say if someone has symptoms and they're mild, a health care provider "may advise a COVID-19 test," and if symptoms are severe, people should contact a health care provider or seek emergency care.
"It is important to realize that you can be infected and spread the virus but feel well and have no symptoms," the updated CDC site says, noting that local public health officials might request asymptomatic "healthy people" be tested, depending on cases and spread in an area.
In its pandemic planning scenarios, the CDC says its current best estimate is that 40% of infections are asymptomatic and 50% of transmission occur before symptoms occur.
Doctors were puzzled by the change and questioned why the CDC did not explain why it made the update.
CDC referred questions to HHS. In a statement provided Wednesday to CNN, HHS Assistant Secretary Dr. Brett Giroir said: "This Guidance has been updated to reflect current evidence and best public health practices, and to further emphasize using CDC-approved prevention strategies to protect yourself, your family, and the most vulnerable of all ages."
HHS has not specified what change in "current evidence" may have driven the change.
The statement continued: "The updated Guidance places an emphasis on testing individuals with symptomatic illness, those with a significant exposure or for vulnerable populations, including residents and staff in nursing homes or long term care facilities, critical infrastructure workers, healthcare workers and first responders, and those individuals (who may be asymptomatic) when prioritized by public health officials."
Wen, the former Baltimore health commissioner, said, "I'm concerned that these recommendations suggest someone who has had substantial exposure to a person with Covid-19 now doesn't need to get tested."
"This is key to contact tracing, especially given that up to 50% of all transmission is due to people who do not have symptoms. One wonders why these guidelines were changed -- is it to justify continued deficit of testing?"
Dr. Carlos del Rio, infectious disease specialist and associate dean of Emory University School of Medicine, told CNN's Jim Sciutto on Wednesday that not testing may be OK in some circumstances -- brief contact, for example.
"But if you have been in contact for 15 minutes and that people doesn't have a mask, I think you need to be tested regardless if you have symptoms or not," he said. "We know especially young people going into the house and then transmit inside the household. So, the guidelines baffle me and I really don't understand them."
Del Rio added that he's concerned about politics influencing these decisions. He noted that President Trump has said in the past that more testing leads to the detection of more cases.
"I am worried that this is just a way to slow down testing and that would clearly be not good," deo Rio said. "We don't want to decrease the amount of testing. We want to decrease cases by decreasing transmission, not by decreasing testing."
A spokesperson at the US Department of Health and Human Services on Tuesday denied the change would affect contact tracing efforts, which most public health officials say is key to any eventual control of the virus. "The updated guidance does not undermine contact tracing or any other types of surveillance testing," the spokesperson said.
HHS said people should consult with their doctors or with local health officials to decide if they need to be tested.
"The guidance fully supports public health surveillance testing, done in a proactive way through federal, state, and local public health officials," the spokesperson said.