After previously encouraging people without symptoms of Covid-19 to get tested if they have been exposed to someone who tested positive, US health authorities have abruptly reversed their position without a clear explanation.

The changes in guidance were quietly made to the website of the Centers for Disease Control and Prevention (CDC) on Monday amid reports in the US media of political interference from the White House.

President Donald Trump has repeatedly said that the US should do less testing, and blamed testing for making it appear as though the country is doing poorly against the pandemic.

This is not true. Though the US is testing at a high level, that is because its outbreak is worse than any other country in the world, with more than 5.8 million confirmed cases and almost 180,000 deaths.

The CDC's site previously said: "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 site now says: "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."

In a statement to CNN, senior health department official 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."

But the health department did not say how the evidence has changed.

The CDC has previously said that between 40-50 percent of people with Covid-19 are asymptomatic and therefore getting testing is important to stop the spread.

Health experts reacted with dismay. 

"I still can't make sense of @CDCgov change in guidance," tweeted Dr Leana Wen, a professor at George Washington University. 

"An estimated 40-50% of people with #covid19 are asymptomatic. Those exposed to the virus need to know to protect their family members & the public. One has to wonder: is this change because we don't have enough tests?"

Both the New York Times and CNN quote officials saying that the CDC was instructed to change its guidelines following pressure from the president.

The Trump administration has previously been accused of pressuring the CDC to change its guidance on reopening schools, weighting its recommendations heavily in favor of in-person learning.

The political neutrality of the Food and Drug Administration was also questioned this week when its chief Stephen Hahn joined Trump in misrepresenting a key statistic about the effectiveness of blood plasma.

He later apologized.