Fourteen London transport workers have now died of coronavirus.
The death toll includes nine bus drivers, three Transport for London workers, an Underground employee and a worker for one of TfL's suppliers, Mayor of London Sadiq Khan told Sky News.
This news comes as a new system of boarding buses is being trialled in London to reduce contact with drivers.
TfL announced it will launch a pilot scheme which will see passengers using the middle door to board, amid growing concern over the safety of drivers.
The trial will be in place from this week on several routes operated by bus firm Abellio out of its depot in Walworth, south-east London, including two that serve hospitals.
Passengers using London buses normally board using the front door and leave using the middle door, to smooth the flow of people.
The switch in policy adds to other new safety measures such as discouraging the use of seats near the driver and covering the holes in their protective screen with clear film.
Sadiq Khan told BBC Breakfast: "What we've done with our buses in London are the most enhanced measures of any buses across the country.
"We're using anti-viral disinfectant, the sort of stuff they use in hospitals to make sure everything is clean including the touch points, steering wheels, handles, garages, rest rooms.
"We've also made sure there is a perspex barrier between the driver and the passengers with a protective film to avoid particles or droplets coming from people's mouth on the driver.
"We've also made sure no passenger can sit near a driver."
The London Mayor added: "We've made sure that all the things that drivers are concerned about are being addressed.
"We're now piloting - in London we've got two doors. One door in and one door out.
"We're piloting on some buses only allowing passengers in through the back door to avoid contact with drivers."
Bus use in London has fallen by around 85% due to the coronavirus pandemic.
TfL director of bus operations Claire Mann said: "London's hard-working transport workers are making a heroic effort at the frontline of the fight against this pandemic, and it is only right we consider everything we can to protect them.
"We've already delivered many other enhanced safety measures and by trialling middle door-only boarding on buses we can gain the information we need to see if we can further improve safety on London's buses.
"Most Londoners can do their bit to protect our bus colleagues and other critical workers by remembering: stay home, don't travel, save lives."
