Race leader Aleksandr Vlasov and Olympic mountain bike champion Tom Pidcock have withdrawn from the Tour de Suisse following positive Covid tests.
Three entire teams – UAE Team Emirates, Alpecin-Fenix and Bahrain-Victorious – also pulled out for the same reason.
And EF Education-EasyPost withdrew four riders, including Rigoberto Uran and Briton Hugh Carthy.
In total, 29 riders had pulled out of the race before the start of Friday’s stage six as a result of the outbreak.
Team Jumbo-Visma were the first team to abandon the race on Thursday.
Following a meeting involving teams, rider representatives and governing body the UCI, Tour de Suisse race director Olivier Senn said the remaining 18 teams had agreed to continue, but he admitted there was no guarantee the race would reach Vaduz on Sunday.
“We’ve very sad that coronavirus is spreading so far within the teams,” said Senn. “But in the moment we keep going as planned.
“It’s obviously not nice. All the teams know the rules. Four teams decided to withdraw, including the one yesterday, so there’s 18 teams left.”
The Tour de Suisse is one of the final major warm-up races before the Tour de France, which runs from 1-24 July.
Nico Denz won stage six on Friday following a photo-finish sprint, while Britain’s Geraint Thomas finished eighth and remains second overall.
Denmark’s Jakob Fuglsang leads the overall standings, despite finishing the 177.5 km stage in 10th place.
