Suburban Cook County will begin vaccinating a fraction of its elderly residents and essential workers on Monday, according to the public health department’s website.

“Update: We will move to phase 1b on Monday, Jan. 25. Vaccine supply is extremely limited and we urge your patience,” a banner on the Cook County Department of Public Health’s website said.

Vaccination started in suburban Cook County last month with phase 1a, which included health care workers and staff and residents of long-term care facilities. Phase 1b in that region includes those 65 or older and front-line essential workers, such as teachers.

The Tribune has asked the county health department for details on how people in 1b can get vaccinated. In Chicago, public health Commissioner Dr. Allison Arwady has predicted most people will get their vaccines from their health care provider, pharmacy or employer. But there also are a number of city-run mass vaccination sites — a model that has been used in the suburbs as well.

For phase 1b in the suburbs, people have been getting the vaccine through similar avenues, including a partnership with Jewel-Osco that served emergency medical services employees. Gov. J.B. Pritzker on Tuesday also announced that four mass vaccination sites in Cook County began vaccinating Illinois residents against COVID-19 as the state readies for the next phase of immunizations.

The sites in Cook County are: North Riverside Health Center, Robbins Health Center, Morton East Adolescent Health Center and Cottage Grove Health Center. The four state sites, which are being stood up with support from the Illinois National Guard, will be open from 9 a.m. to 5 p.m. Monday through Saturday, Pritzker’s office said.

The sites are currently open to front-line health care workers seeking vaccinations, until Illinois officially moves into phase 1b of its vaccination plan on Monday.

In suburban Cook County, 102,242 vaccine doses have gone into arms as of Thursday, according to the Illinois Department of Public Health. One percent of its 2.5 million population is fully vaccinated.

Hospitals in the suburbs that had leftover doses were allowed to start vaccinating its highest need patients in phase 1b starting this week. The full transition to phase 1b does not mean phase 1a has finished, according to CCDPH’s website, and the latter population will be prioritized. The website estimates about 375,000 people 65 or older and 270,000 front-line essential workers under the department’s jurisdiction.

“Vaccine supply is expected to continually increase in the weeks and months ahead,” the website says. “CCDPH will work with partners to ensure those that are the most vulnerable have access COVID vaccinations. Eventually, the vaccine will be available to all suburban Cook County residents who want it.”

CCDPH’s jurisdiction spans the Cook County suburbs with the exception of Evanston, Skokie, Stickney Township and Oak Park. According to a city newsletter, Evanston began phase 1b this week starting with first respon

ders and people 85 or older. Those 75 and older may start getting inoculations next week, followed by those 65 or older, people in congregate care or shelter sites and other frontline essential workers.

Skokie officials have said they hope to move on from phase 1a after this week. At least one Skokie school district has begun vaccinating teachers. School District 68 Superintendent James Garwood said the first day for its educators to get the shot was Tuesday, and he hopes all of his district’s eligible staffers under 1b who want the vaccine can get it by the end of March.

In Stickney, phase 1b will begin Monday, while Oak Park’s health department website is asking for “patience” because it does not yet have a time frame for the next stage.

Chicago Tribune’s Jamie Munks contributed.

ayin@chicagotribune.com