Beginning Wednesday this week, the Steamship Authority will require all employees to show they have had at least one Covid-19 vaccination, the boat line announced Tuesday.

The vaccination requirement takes effect Jan. 5, which is also the first day of the winter ferry schedule. Employees who are not fully vaccinated will have until Feb. 16 to come into compliance.

The SSA is using the Centers for Disease Control and Prevention definition of fully vaccinated, as adopted by the Massachusetts Department of Public Health, the announcement said.

Speaking to the Gazette by phone Tuesday, SSA spokesman Sean Driscoll said the vaccine mandate has been under discussion among boat line managers and with the numerous maritime unions that cover employees since October.

Gov. Charlie Baker issued a vaccine mandate late in the summer for all state employees that was subsequently challenged. Because the SSA is a quasi-state agency, Mr. Driscoll said it was unclear whether the mandate applied, but in the end he said it was decided that the boat line should adopt its own mandate.

“It really comes down to a matter of public health — it’s for the protection of our employees, it’s for the protection of our customers,” Mr. Driscoll said.

The boat line employs roughly 700 people, 500 of them permanent, year-round workers; the rest are seasonal or part time employees. The total vaccination rate among employees is around 65 per cent, Mr. Driscoll said.

There are eight maritime unions, the majority of them Teamsters, according to Mr. Driscoll. To date only one has agreed to the vaccine mandate: the Service Employees International Union, which represents reservation clerks.

Negotiations are ongoing with the other seven bargaining units, but Mr. Driscoll said the requirement is nevertheless being implemented as a management mandate and applies to all employees, he said.

He said since the notice went out to employees, many have been asking questions, including about medical and religious exemptions. Others have submitted proof of vaccination.

“It hasn’t gone unnoticed,” Mr. Driscoll said of the new requirement. “Our HR department has been very busy.”

Employees who are not fully vaccinated by Feb. 16 will be subject to progressive discipline, including possible termination, the earlier announcement said. An incentive of up to $500 per employee is also being offered to encourage vaccination.

A mask requirement is already in effect on all SSA ferries and applies to traveling customers as well as employees. The face covering mandate is a U.S. Coast Guard regulation.