Earlier he said: “Hope Hicks, who has been working so hard without even taking a small break, has just tested positive for COVID 19.
“Terrible! The first lady and I are waiting for our test results. In the meantime, we will begin our quarantine process!”
Trump and his wife spent considerable time with Hicks, a counsellor to the President, this week.
During an interview earlier with Fox News host Sean Hannity, Trump said: “Whether we quarantine or whether we have it, I don’t know.
“I just went for a test and we’ll see what happens.”
Hicks travelled with Trump aboard Air Force One to and from the presidential debate on Tuesday and accompanied him to a rally in Minnesota the following day.
Bloomberg initially reported Hicks’ positive test result.
Hicks was the latest person in Trump’s orbit to contract the virus, which has infected more than 7.2 million Americans and killed more than 200,000. Other senior staff have contracted COVID-19 and recovered, including National Security Adviser Robert O’Brien, but few spend as much time with the President as Hicks, whose service dates to his 2016 campaign.
Some people close to Hicks were told that she was experiencing symptoms of the disease.
Trump’s decision to self-quarantine could dramatically curtail his campaign schedule with a month remaining until the November 3 election.
The Centres for Disease Control and Prevention, the nation’s top public health body, recommends that people quarantine for 14 days if they have come into close contact with someone who has contracted COVID-19.
Trump has a campaign rally scheduled on Saturday (AEST) in Florida that now appears unlikely to go ahead.
The next presidential debate is scheduled on October 15, a fortnight away.
Trump rarely wears a mask and has belittled his re-election challenger, Joe Biden, for routinely covering his face. He has also resumed holding large campaign rallies at which thousands of his supporters gather, shoulder-to-shoulder, few in masks.
Most of the events are held outdoors at airports but two recent events – in Arizona and Nevada – were indoors, a setting that public health experts say raises the risk of virus transmission.
At the debate in Cleveland several members of Trump’s entourage entered the debate hall without face coverings, or removed them as they sat down, and refused an offer of masks from a doctor at the Cleveland Clinic, which was co-hosting the event.
Biden’s guests wore masks.
Hicks was seen on Tuesday riding without a mask in a staff van with White House senior adviser Stephen Miller, campaign adviser Jason Miller and others.
When they returned to Washington, Stephen Miller and Hicks were seen sharing an umbrella as they exited Air Force One in the rain.
Miller’s wife, Katie Miller – Vice-President Mike Pence’s press secretary – recovered from COVID-19 earlier this year.
Matthew Knott is North America correspondent for The Sydney Morning Herald and The Age.