Sadiq Khan confirmed 21 transport workers have died after testing positive for coronavirus. They include 15 bus workers.
London's Mayor said he was doing everything he could "to keep transport workers and passengers safe".
Unions have called on Transport for London to provide masks and gloves for Tube and bus drivers.
Mr Khan said TfL was trying to find supplies of personal protective equipment.
Enhanced cleaning has already been rolled out across the network and protective film has been fitted to bus driver cabs. A different way of boarding buses using the back door is also being trialled on some routes.