The 55-year-old Johnson was released from the hospital Sunday three days after being transferred out of intensive care. He'd spent three nights in the ICU fighting the COVID-19 infection.
The British leader was taken to the hospital on April 5 after 10 days of self-quarantine and "mild" symptoms.
Spokesman James Slack told reporters Monday the parting test was "standard practice" and that Johnson was recovering at Chequers, a 16th-century countryside mansion reserved for sitting prime ministers, with his pregnant fiancee Carrie Symonds.
Slack said Johnson is not "currently carrying out government work" and is "focusing on his recovery."
It's unclear when he will return to work as Britain's official tally of coronavirus deaths surpassed 11,000 -- the most of any European country.
Symonds thanked members of Britain's National Health Service for saving his life, praising the doctors and nurses at St. Thomas' Hospital in London as "incredible" in a Twitter message Sunday.
"I will never, ever be able to repay you and I will never stop thanking you," she wrote. "There were times last week that were very dark indeed. My heart goes out to all those in similar situations, worried sick about their loved ones."
Appearing in a video posted to social media Sunday, a much healthier Johnson also lavished praise on doctors and nurses, saying his life-or-death battle "could have gone either way." He said NHS workers "kept putting themselves in harm's way, kept risking this deadly virus.
"It is thanks to that courage, that devotion, that duty and that love that our NHS has been unbeatable."
The death rate has become so overwhelming that hospitals are running out of body bags to transport the dead, BBC News reported. Healthcare workers said bodies have instead been wrapped in sheets. Medical suppliers reported difficulty in finding sources for zipped mortuary bags -- one blamed the situation on stockpiling by buyers.
Funeral directors, like front-line medical staff, are also facing shortages of personal protective equipment as the mortuary industry strains to keep up with the death toll.