Former New Jersey Governor Chris Christie has urged Americans to take coronavirus "seriously" after spending days in intensive care with Covid-19.
Mr Christie, a Trump administration ally, revealed on Thursday he had recovered from the disease.
He was one of several virus cases confirmed at the same time as President Donald Trump in early October.
The infections have been linked to a "superspreader event" at the White House.
More than a dozen cases have been traced to the Rose Garden event on 26 September, including two senators, the White House press secretary and President Trump's former counsellor Kellyanne Conway. All appear to have recovered.
Mr Christie said he attended the event, a ceremony where Mr Trump formally announced his nomination of the conservative Judge Amy Coney Barrett for a Supreme Court vacancy, believing he had "entered a safe zone".
"I was wrong," Mr Christie, who is in a high-risk category for Covid-19 because of his weight and asthma, said in a statement.
"I was wrong to not wear a mask at the Amy Coney Barrett announcement and I was wrong not to wear a mask at my multiple debate prep sessions with the President and the rest of the team."
Mr Christie said he hoped his experience would encourage Americans to follow virus guidelines "in public no matter where you are and wear a mask to protect yourself and others".
By contrast, Mr Trump left hospital after three nights of treatment for Covid-19, urging Americans not to let the virus "dominate" them.
Mr Christie, 58, was admitted to the Morristown Medical Center in New Jersey on 3 October as a precaution.
He said he ended up spending seven days in the intensive care unit of the hospital. He said he recovered thanks to the "skilful care" of doctors and "extraordinary treatments", including an antibody cocktail given to President Trump.
Mr Christie said his stint in "isolation" gave him time to do some thinking about the coronavirus pandemic.
"It is something to take very seriously," Mr Christie said of the virus, which has killed more than 217,000 people in the US to date.
"The ramifications are wildly random and potentially deadly. No one should be happy to get the virus and no one should be cavalier about being infected or infecting others."
Who is Chris Christie?
- A Republican politician who served as the 55th Governor of New Jersey from 2010 to 2018
- A former US Attorney for New Jersey, a position he held from 2002 to 2008 under President George W Bush
- The former chairman of Mr Trump's presidential transition team in 2016 until shortly after his election victory
- An adviser to President Trump ahead of his first TV debate with Democratic presidential rival Joe Biden in October
In his first TV interview since contracting Covid-19, Mr Christie called on President Trump to encourage the wearing of face masks among Americans.
Mr Trump expressed approval of masks at a televised town-hall event on Thursday, as he faced a grilling from the public before he takes on Democratic challenger Joe Biden in November's presidential election.
But Mr Christie said that President Trump's messaging wasn't explicit enough, telling ABC's Good Morning America: "I think we should be even more affirmative about it. That's why I put out the statement I did."
.@ABC NEWS EXCLUSIVE: @GovChristie talks to our @GStephanopoulos for the first time since he tested positive for coronavirus. “I let my guard down for a couple of days inside the White House grounds and it cost me in a significant way.” https://t.co/aYZI2LnNcV pic.twitter.com/h12I8IpPjj— Good Morning America (@GMA) October 16, 2020
In his statement, the former governor said: "Every public official, regardless of party or position, should advocate for every American to wear a mask in public, appropriately socially distance and to wash your hands frequently every day."
Masks have turned into a divisive political issue in the US. President Trump has previously mocked Mr Biden and some journalists for wearing masks, and many of his supporters have attended his rallies without face coverings.
In contrast, Mr Biden has taken a cautious approach to coronavirus during the election campaign, regularly wearing a mask in public while urging others to do so.