Biden, who received the revival of Covid last month, traveled from Washington to the G-20 summit in Italy on Thursday.
“I last saw the president on Tuesday when we were sitting outside more than six feet apart and wearing masks,” she added.
She said she was negative every day from Wednesday to Saturday before being positive on Sunday.
“Thanks to the vaccine, I only had mild symptoms that allowed me to continue working from home,” Psaki said in her statement.
She said she would resume work after a 10-day quarantine period after a negative rapid test.