Five London bus workers have now died with Covid-19, according to a trade union.
Unite said the deaths were a "terrible tragedy".
Regional secretary Peter Kavanagh said: "Unite will assist the families of our members in every possible way during this terrible time.
"Unite has been working continuously with Transport for London (TfL) and the operators to ensure the safety of drivers and others in the industry who are performing a heroic job in getting NHS and care workers to their places of work.
— Unite the union (@unitetheunion) April 4, 2020
"These measures include deep cleaning of buses, additional cleaning of touch points, the sealing of screens around the driver, the provision of hand sanitiser for all and placing the passenger seating closest to the driver out of bounds.
"I have been in direct contact with the Mayor of London who shares our view that bus drivers must be fully protected.
"My officers are holding daily meetings with TfL, exploring further safety improvements and we are absolutely committed to doing everything in our power to make the driving of buses safe during this unprecedented crisis.
"We are also calling on the Government to make provisions for transport workers in terms of personal protective equipment."