Governor Charlie Baker is ordering the state’s 42,000 executive branch employees to be vaccinated, or face potential termination.

All employees will be required to provide proof of having been vaccinated by October 17. The new mandate applies to employees working remotely and those in-person.

Senate President Karen Spilka said she applauded the governor for "issuing a strong vaccination mandate," and said it "underscores the message that vaccination is our best tool for ending the disruption and suffering" caused by the pandemic.

Spilka said in a press release that the Senate's own COVID-19 working group will soon issue recommendations related to a vaccine mandate for the body's own employees.

Baker first mentioned the possibility of the order on GBH News on Wednesday and issued the order on Thursday afternoon.

"Executive Department employees who are not vaccinated or approved for an exemption as of Oct. 17, 2021 will be subject to disciplinary action, up to and including termination," the administration said in a press release on Thursday.

Executive offices for the state encompass prison guards, social workers, people who work for the Office for Administration and Finance, Office of Education, Office of Energy and Environmental Affairs, Health and Human Service and others.

SEIU 509, the union that represents over 8,000 state workers, said it "unequivocally supports" the mandate, and will bargain with the state to "ensure its members' rights are protected."

"As health and human service workers, we’ve seen the effects of the COVID pandemic up close, and we know mandates are the best decision for public health," said Peter MacKinnon and Israel Pierre, President and Treasurer of the union in a joint press statement.

MacKinnon told GBH News in a Zoom interview that the union's 70-plus member executive board voted last week in favor of asking all employers to issue vaccine mandates, as long as there are safeguards for employees. He said the union estimates about 75% to 85% of its members have gotten a vaccine and hopes more do.

“Making sure everyone has access to it. Equity for the — to the vaccine is going to be key. Making sure nobody is punished because they’re not able to immediately get it but they have the ability to access it,” he said. “Those things seem to be built in and that's what Local 509 wants to see.”

The Massachusetts Correction Officers' Federated Union, which represents guards and officials in the state's prison system, said it "does not agree with a forced vaccination" and is pursuing legal options against the governor's office.

"We're deeply disappointed in this Executive Order," said the union's executive board in an online memo to members. "We feel this Executive Order fails to uphold your individual rights and is unconstitutional."

The memo goes on to say MCOFU is pursuing "all legal and legislative remedies," which could include an injunction in court.

Baker earlier this year had resisted calls to mandate vaccines for state workers.