ABC News reported that Guilfoyle was asymptomatic.
Trump, Jr. tested negative, but both will self-isolate, Trump Victory Finance Committee Chief of Staff Sergio Gor said.
He said she will be retested to confirm the diagnosis because she isn't showing any symptoms of COVID-19, the disease the virus causes.
All events featuring Guilfoyle, the national chair of Trump Victory, were being canceled, Gor said.
The news was revealed Friday night by the New York Times' Maggie Haberman.
Indonesia seeking its own COVID-19 vaccine amid worry about access
In fact, the registry details show that the phases are envisaged to be completed in 194 days or over 6 months. Globally, over 100 candidates are being tested on humans, but no vaccine has yet been approved.
Don Jr. and Guilfoyle also attended a packed Hamptons party on Saturday, June 27.
A source told Page Six that the packed get together looked 'as if COVID had never happened'.
Trump has been criticized by some for holding rallies without social distancing or mask requirements amid the pandemic.
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American Twitch streamer and content creator Byron "Reckful" Bernstein has passed away at 31 years old, according to sources. Reckful was a former World of Warcraft and Hearthstone professional who represented Complexity between 2009-2011.
Guilfoyle was tested for the virus, along with many others who could come in close contact with the president. In May, one of Trump's personal valets tested positive, as did Vice President Mike Pence's press secretary. She's married to Stephen Miller, a top Trump aide.
Masks were not required at the Make America Great Again rally in Tulsa, Oklahoma and won't be at the Fourth of July event. Out of caution, the couple plans to drive back from South Dakota to the East Coast, the person said.
In posting the image, Trump Jr. wrote, "Solid point". At the time - as now - New York State was in Phase 3 of reopening; outdoor gatherings of up to 25 people are permitted, so long as masks are worn when distances of six feet can not be maintained.
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Meanwhile, the government reported that U.S. unemployment fell to 11.1pc in June as the economy added a solid 4.8 million jobs. The UAE reported 400 new cases of the coronavirus on Thursday, along with 504 recoveries and one death.