State and Cook County officials debuted a mass vaccination site in Forest Park on Monday that, come Friday, will allow some 1,000 individuals to get the coronavirus vaccine each day.

But, as with so much else in life, it comes with a catch.

“It will serve as yet another mass vaccination site open up to all eligible residents, no matter their zip code,” Gov. J.B. Pritzker said of the site’s soon-to-come launch.

Only those who are eligible.

If only it were that simple.

Who the state considers eligible for the vaccine — and when they’re eligible — is different from Cook County’s considerations, which, in turn, is different from Chicago’s.

Furthermore, Chicago’s age, employment and health condition limitations may be disregarded if residents live in certain zip codes; any adult who resides in a neighborhood deemed by the city to be the most vulnerable to COVID-19 may get the vaccine, regardless of whether they’d otherwise meet the qualifications.

The alphabet soup of rolling eligibility is further clouded given that municipalities are like Russian dolls: Chicago is nested within Cook County, which is nested within Illinois.

Which is why, for example, Walgreen’s explanation that “eligibility of who can receive a vaccine is determined by the jurisdiction and we along with all pharmacy partners follow the jurisdiction guidance that is given” is in itself complicated.

A general rule: The strictest eligibility rules may be in effect. So assume that a pharmacy in Chicago will go by the city’s guidelines, while Cook County’s suburban mass vaccination sites will go by Cook County’s eligibility rules. Locations outside of the collar counties may be using the state’s looser guidelines.

Adding to the complexity, there’s no single government-run website where an Illinois resident can sign up.

Colorado resident Nick Muerdter created a free website – as a side project, no less — to help individuals navigate the vaccine web.

VaccineSpotter.org scans pharmacy websites for vacant appointments, but his website does not scan for eligibility. That’s up to the user and the vaccine provider to sort out.

“I feel like I struggled with how maybe this influences underserved areas and people’s access to technology and age ranges and stuff like that. But I hope that just by putting the information out there that however people are getting vaccines – if they’re getting help from other people, I know there’s a lot of volunteer organizations that are doing that type of thing to help people and I know a lot of them are using the website – at the end of the day, I hope it’s helping,” he said.

Eligibility is broken into “phases”: 1A, 1B, 1B Plus (a category Chicago is skipping entirely) and 1C.

All of the state is – and has been – in Phases 1A and 1B: The former covers health care workers, those who work in nursing homes and residents of long-term care facilities. The latter covers individuals ages 65 and up, as well as front-line workers like grocery store clerks, mail carriers, public transit employees and teachers.

Past those initial phases, things deviate.

Illinois entered 1B Plus on Feb. 25, while Cook County just expanded to 1B Plus on Monday; the city of Chicago is bypassing this phase entirely.

For Cook County and Illinois, 1B covers individuals ages 16 and older who are disabled, pregnant, obese, who smoke or who have health conditions like cancer, diabetes, heart conditions, obesity and sickle cell disease.

The city of Chicago extends vaccine eligibility to individuals with high-risk medical health conditions (but not smoking) in its 1C phase, which begins March 29.

Nonessential front-line workers in Chicago are also part of 1C, a category that the city’s top doctor, Dr. Allison Arwady, acknowledged as one in which a majority of Chicago residents will become eligible for the vaccine.

Cook County has not announced when it will move to 1C.

But workers of various stripes are eligible sooner under Illinois’ rules.

The state on Monday began allowing government employees, members of the media and educators in higher education to get vaccinated.

Illinois broadens 1C to all other essential workers at the end of the month; come March 29, Illinois’ eligibility extends to restaurant staff, bartenders, hairdressers, those in the construction trades and clergy.

But it won’t be much longer before any adult can get the vaccine, via Illinois’ rules (it’s unclear when Chicago and Cook County will catch up).

Illinois is opening up eligibility wide open on April 12.

While officials on Monday continued to ask for patience with the rollout, Pritzker signaled there’s another vaccine date individuals may want to circle next Monday: The federal government has signaled that “millions” of doses of the Johnson & Johnson vaccine are expected to be shipped to Illinois then, he said, on top of the Moderna and Pfizer allotments the state currently receives.

Follow Amanda Vinicky on Twitter: @AmandaVinicky