The Yell Sound ferry service will be disrupted until the weekend, the SIC has said.
The final sailing from Ulsta will be at 6.30pm, and from Toft at 6.55pm, tonight (Wednesday), and on Thursday and Friday nights.
Environment and transport chairwoman Moraig Lyall said the council “simply do not have any available staff” able to stand in for sick workers.
“Passenger and crew safety has to be our principal priority,” she said.
“I know how dismaying and disruptive this will be, especially to island residents, and our officers have been working hard to try to find an alternative approach.
“We expect the service to return to normal at the weekend.
“I want to acknowledge the patience shown by the travelling public, and thank our staff who are working very hard in difficult circumstances.”