"Fears" that Formula 1 will not be able to race through its currently-scheduled 2021 calendar "are justified".
That is the view of Toto Wolff, who admits that with the coronavirus pandemic still raging, the sport will have to continue to adjust and adapt for now.
Another boss worried about the situation is Beat Zehnder, Alfa Romeo's team manager.
"Personally, I believe we will have to take the season step by step after the start in Bahrain," Zehnder told Germany's Auto Motor und Sport.
"As things stand today, we could go to Imola but I cannot imagine that the government in England will let us do a grand prix with the current infection situation there.
"As of today, we probably couldn't go to the Nurburgring either. The whole situation is very volatile," he added.
"The vaccines will work at some point, but it's not happening as quickly as we all hoped. I think that will only help us in the second half of the season.
"At the moment I don't see that we can drive on the temporary tracks, even if Monte Carlo has assured us that they want to run the race," Zehnder added.
"If you consider that we still have to develop the 2021 car a bit while doing 23 races and preparing for 2022 at full throttle - it will be a tough year."
Mercedes boss Wolff says those like Zehnder who are beginning to publicly worry about Formula 1's 2021 season should be listened to.
"Of course the fears about the calendar are justified," he told the Austrian broadcaster ORF.
"Australia has already been postponed, and because that race is very important it was decided to postpone it rather than cancel it," Wolff said.
"In China, large events are generally prohibited until August. We cannot influence these things in any way, which is why the season is starting in Bahrain - we know the track well so it's relatively easy to start there.
"At the moment I guess we will be able to have a normal season only after the summer," he added.
