Six months after Mr. Johnson was released from the hospital, he has yet to shake off questions about the effects of the disease on his energy, focus and spirit. His health is a source of whispered speculation in the hallways of Parliament, questions from reporters and ominous musings by columnists, for whom Mr. Johnson’s illness has become a symptom of his broader political decline.

“It’s a metaphor for his government, and that’s affecting him personally,” said Jonathan Powell, who was chief of staff to Tony Blair when he was prime minister. “He looks like the wrong man for the job at this time.”…

Despite his efforts, Mr. Johnson has never recaptured the public buoyancy that propelled him to a landslide election victory last December. On Tuesday, speaking to his Conservative Party’s annual conference, he tried again to put to rest questions about lingering effects of Covid-19, the disease caused by the virus.

“I have read a lot of nonsense recently about how my own bout of Covid has somehow robbed me of my mojo,” the prime minister said in a tone of theatrical umbrage. “And of course, this is self-evident drivel, the kind of seditious propaganda that you would expect from people who don’t want this government to succeed.”