US health officials have sparked criticism and confusion after posting guidelines on coronavirus testing from the White House task force that run counter to what scientists say is necessary to control the pandemic.

The new guidance says it is not necessary for people who don’t feel sick but have been in close contact with infected people to get tested. It was posted earlier this week on the website of the Centers for Disease Control and Prevention.

The CDC previously had advised local health departments to test people who have been within 6 feet of an infected person for more than 15 minutes. But on Monday a CDC testing overview page was changed to say that testing is no longer recommended for symptom-less people who were in close contact situations.

There was a caveat, however. Testing may be recommended for those with health problems that make them more likely to suffer severe illness from an infection, or if their doctor or local state officials advise they get tested.

CDC officials referred all questions to the agency’s parent organisation, the US Department of Health and Human Services in Washington, DC.

That suggests that HHS ordered the change, not CDC, said Jennifer Nuzzo, a Johns Hopkins University public health researcher.

After twitter lit up with head-scratching and alarm about the change, HHS officials sent an email on Wednesday saying the guidance was revised to reflect current evidence and the best public health interventions, but did not detail what the new evidence was.

The officials also said they wanted to emphasise that people should follow measures designed to help stop spread of the virus, like wearing a mask, staying at least 6 feet apart from others and washing hands.

The decision came out of meetings of a White House coronavirus task force, HHS officials said.

In a call with reporters on Wednesday, Dr Brett Giroir, the HHS assistant secretary for health, said the guidance language came from the CDC.

But he also said many people were involved in lots of editing, lots of input.” He said federal officials achieved consensus but it was difficult to attribute the final language to any one source.

Its kind of hard to know how much was written by what person, Giroir said.

Across the country, public health experts called the change bizarre. They noted that testing contacts of infected people is a core element of public health efforts to keep outbreaks in check, and that a large percentage of infected people — the CDC has said as many as 40 per cent — exhibit no symptoms.

“I was taken aback and didn’t know that it was under consideration,” said John Auerbach, president of Trust for America’s Health, a non-profit that works to improve US preparedness against disease.

The recommendation not to test asymptomatic people who likely have been exposed is not in accord with the science. Ultimately, restricting testing could be self-defeating, because it could skew the numbers and create a perception that rates of infection are higher.

Testing people who appear to be healthy would tend to lower the overall rate of positive results, while narrowing testing to people who are sick would raise the overall positive rate, Auerbach said.

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