The man was one of two people injured when they were hit by a car suspected to have been driven by a teenager who was over the drink-drive limit in the early hours.
While the suspect remains in custody, officers have appealed for help tracking down witnesses who helped at the scene following the crash in Holloway Circus Queensway just before 3.40am today.
West Midlands Police believe there were a handful of people who could be at risk after coming into close contact with the man before emergency services arrived.
The man remains in hospital with serious head injuries and has tested positive for Covid-19.
A woman - also a pedestrian and known to the man - suffered a leg injury. It is not yet known whether she is also infected.
A police officer who was in close proximity is now self-isolating.
A spokesman for West Midlands Police said said: "We urgently need the members of the public who were in contact with the man and woman after the collision at around 3.40am to identify themselves.
"They are not only witnesses, but importantly need to follow Government guidelines when in close contact with confirmed Covid-19 cases."
An 18-year-old man arrested on suspicion of dangerous and drink driving remains in police custody for questioning.
The road was closed for around six hours as a result, causing heavy congestion and delays throughout the city centre, but is now back open.
A white Vauxhall with major damage to its windscreen was left behind the police tape as collision investigators examined the scene.
An 18-year-old has been arrested on suspicion of dangerous and drink driving and was taken into custody for questioning.
The man and woman both suffered serious injuries and were taken to the city's Queen Elizabeth Hospital.
A spokesman for West Midlands Ambulance Service said: "When ambulance staff arrived at the scene, they found two pedestrians, a man and a woman, who had reportedly been in collision with a vehicle.
"The man was in a serious condition and ambulance staff worked together to administer advanced trauma car before conveying him on alert to Queen Elizabeth Hospital Birmingham.
“The woman had sustained serious injuries which required treatment on scene before she was taken by land ambulance on blue lights to the same hospital for further emergency care.”