The results in Selby and Uxbridge threw up as many questions as answers for Labour.
The Conservatives lost two other seats last night and held Uxbridge by just 495 votes.
There was speculation Johnson might perform a “chicken run” to a safer seat ahead of the next general election.
The 33-year-old was rushed to hospital with "critical injuries" after the incident in London, police said.
The PM broke an election tradition by voting in Westminster, rather than in his Uxbridge and Ruislip South constituency.
Focus group in Boris Johnson's seat likens free broadband pledge to communist Cuba and see Corbyn as "indecisive"
A focus group in Uxbridge showed how the opposition could still succeed, Edelman's public affairs MD James Morris writes.
A 17-year-old boy has been arrested on suspicion of murder.
'A few weeks ago this would have sounded fantastical: now it is political reality.'