One mass vaccination site in the Chicago suburbs has been so overwhelmed that the first slots were quickly snapped up and openings have remained scarce, even with tightened eligibility rules.

More than 200 miles away, a site on Illinois’ western border has plenty of openings and regularly welcomes any state resident who qualifies for a shot.

Three months into Illinois’ mass vaccination program, the striking differences between those two sites — in northwest suburban Des Plaines and downstate Quincy — illustrate the frustratingly uneven pace of vaccinations across the state.

Quincy is in Adams County, where 27% of its residents were fully vaccinated as of Friday, the best rate in Illinois. In suburban Cook County, the rate is half that, and it’s even worse in most of the collar counties.

Roughly 13% of Illinois’ entire population is now fully vaccinated, placing Illinois first among the 10 largest states based on the most recent federal data. That’s an improvement from the early stages of the vaccination campaign, when a previous Tribune review found Illinois was struggling to keep pace with most states.

But disparities continue to plague the effort. Within Chicago, a map of the most vaccinated neighborhoods and a map of the areas hardest-hit by COVID-19 are near opposites. Illinois also has seen a large racial divide in vaccination, with a higher percentage of white residents vaccinated than Black and Hispanic residents.

Now, state data also shows an emerging geographic divide, with the highest vaccination rate seen for Illinois residents living outside the Chicago area, a group that was 16% fully vaccinated as of Friday. The next highest rate is in suburban Cook County with 14%, which is outperforming the collar counties’ 12%. The city of Chicago now has the lowest rate, 11%, after falling slightly behind the collars.

There are no easy explanations for these differences, in part because the process of distributing doses remains mired in confusion and secrecy. The state released some data showing that some counties, including Adams, received far more doses than others in proportion to population size. That helped those areas boost their vaccination rates.

The disparities in supply and demand have prompted sizable numbers of people to travel across county lines, at times driving for hours, to get shots in places with more availability. So many suburbanites were signing up for shots at the United Center in Chicago that city officials began limiting slots based on where people live, favoring city residents from hard-hit ZIP codes.

The Pritzker administration acknowledges geographic disparities but said officials are doing their best to manage them. That’s one reason the state is opening more mass vaccination sites, including in the collar counties, and setting up temporary clinics that travel from place to place.

Illinois Department of Public Health Deputy Director Andrew Friend told state senators Monday that the agency is trying “to get the state to move together through these vaccination periods.”

Measuring the pace of vaccinations is complicated by the fact that some vaccines require two doses. The Tribune based its analysis on the number of people fully vaccinated — who have gotten either the single Johnson & Johnson shot, or both doses of the Pfizer or Moderna vaccines.

The analysis found that, while the race/ethnicity gap has been evident from the beginning of the vaccination campaign, the geographic gap emerged after Illinois expanded vaccinations to people over 65 and front-line essential workers — in essence, about a fourth of the population.

The geographic disparity grew wider after the governor added a phase “1b plus” to include people at least 16 years old who have preexisting conditions. (Chicago and some nearby counties are only now preparing to offer vaccines to that group, citing backlogs that needed to be addressed first.)

A deeper look into different counties shows just how big the geographic gap has gotten.

Adams County hugs the Mississippi River and is about a five-hour drive from Chicago’s Loop. As of Friday, 27% of the county’s 65,000 residents, of any age, were fully vaccinated. Based on researchers’ population estimates, about 48% of people who qualify for shots under the state’s guidelines got their shots. For senior citizens, a key demographic in the 1b group, 65% were fully vaccinated.

In Champaign County, 21% of all residents are fully vaccinated (including 45% of qualifying residents and 66% of seniors). And in Peoria County, it’s 18% of residents (30% of qualifying residents and 63% of seniors).

But other counties show vaccination rates less than half those achieved in Adams, Champaign and Peoria counties. In Lake County, for example, just 11% of residents were fully vaccinated by Friday (including 24% of qualifying residents and 31% of seniors).

The Tribune posts daily updates by county that show higher vaccination rates in west-central Illinois and pockets of higher rates elsewhere, generally outside the Chicago area.

Even within the Chicago region, there are unexplained divides.

Residents in Lake, Kane, McHenry and Will counties have not kept pace with suburban Cook or DuPage County. Chicago had been keeping up with those areas until March, but has since fallen to levels near the other collar counties.

Another way to look at the disparities is by comparing two places that are similar in some respects yet show vast differences in vaccination rates.

Take Chicago’s 60636 ZIP code, most of which is West Englewood, a majority Black neighborhood that Chicago has rated as the city’s most affected by and most vulnerable to the pandemic. The state deems mostly white Adams County as among the least vulnerable counties. But the populations in both areas include nearly identical percentages of front-line essential workers and senior citizens.

As of Friday, just 5% of the residents of West Englewood’s ZIP code had been fully vaccinated.

Adams County had reached that vaccination rate by Feb. 9.

It’s difficult to untangle who’s getting doses, how many and when.

The federal government ships some doses directly to Illinois providers, but the bulk of shipments come through the state health department — which controls doses outside Chicago — and the Chicago health department, which gets its own supply.

The state has several different ways it ships doses across Illinois — including increasingly through mass vaccination sites staffed by the National Guard. But the vast majority of shots move through the nearly 100 local health departments. Those departments, in turn, host clinics and dole out doses to other providers.

One of the universal complaints of local health officials has been the uneven supply of vaccine, mixed with uncertainty over how the state figures out how much to send where.

“Even we don’t know exactly how those allocation decisions are made,” said Chris Hoff, DuPage County’s director of community health resources.

In Will County, officials are expecting to get enough vaccine to open two more clinics that will triple vaccinations. But four weeks ago, they had to temporarily shutter a clinic at Joliet West High School because of lack of supply. Rows of tables had been set up near the indoor track to vaccinate 6,000 seniors that week, but the county didn’t get enough doses.

“We’ve got the sites. We’ve got the supplies ordered, we’ve got the volunteers and personnel. We just need the vaccine,” county health department spokesman Steve Brandy told the Tribune at the time.

Figuring out the flow of vaccine supplies has been nearly impossible for the public. On Dec. 31, the Tribune asked the state for detailed data that tracked every shipment. The state declined at first, then gave the Tribune data on detailed shipments through Feb. 15 after the Tribune threatened a lawsuit.

A Tribune analysis of that data showed that IDPH directed more doses to some counties than others relative to population size. The counties that got more doses per capita, perhaps not surprisingly, have achieved higher vaccination rates.

Take Adams County again, which by Feb. 15 led the state by getting 148 doses per 100 residents eligible for phases 1a and 1b. That’s based on a Tribune analysis of the provided data, and population estimates by a joint group of public and private researchers used by some governments, including Illinois.

Peoria and Champaign counties were also among the top five getting doses, at rates of 123 and 110 per 100 1b-qualified residents, respectively.

By comparison, the suburban counties ranged from a rate of 39 (in Lake County) to 62 (in suburban Cook County), according to the analysis.

When asked about the differences, the state noted that some counties, including Adams, were acting as regional vaccination hubs for residents of nearby counties, and therefore received larger allocations. But that doesn’t explain why more of the state’s vaccine supply wasn’t shipped to counties where demand is higher and vaccination rates are lower.

The state recently provided newer data to the Tribune showing allocations evening out statewide. That includes figures released online for the first time Wednesday that suggest distributions are now based solely on each county’s population.

Still, the Tribune found vaccination trends were set in the early months, when there were noticeable differences among the counties in per-capita dose distribution.

And in Adams County, the early vaccination successes meant officials were rewarded with even more supply. While touring the vaccination site at the Quincy convention center Feb. 10, Pritzker said his health department had been feeding the county “extra doses … because of its faster pace.”

“From the very beginning the work here has been exemplary. You make our state proud,” Pritzker told local officials.

State officials said they have been adjusting the distribution of doses throughout the rollout.

Pritzker spokeswoman Jordan Abudayyeh said in a statement that the state “closely monitors the progress among local health departments and directs additional vaccine to counties with high throughput to ensure doses are being administered quickly, instead of sitting on shelves in counties with large inventories.”

But there is no set standard for what is an acceptable pace.

Instead, the administration said it studies the kind of inventory data it posts online, questioning counties if officials see backlogs of unused doses.

Some collar counties “have had large quantities of vaccine sitting in inventory as IDPH has coached them through various issues with their providers,” Abudayyeh said.

One example the state cited was Metro Infectious Disease Consultants, a medical practice based in Burr Ridge where thousands of doses have been sitting in freezers for weeks.

The practice’s managing partner, Dr. Russell Petrak, said that’s because he is saving half his doses to use on people who return for their second shot a few weeks after their first. Health officials did not tell him, he said, that the state’s vaccination plan directs providers to “use the entire dose allocation and not hold first doses of vaccine back.” Officials say shipping second doses separately ensures a faster flow of shots.

Petrak said he has ethical concerns about that policy and his office wouldn’t have signed up to administer vaccines if he had known.

The state provided the Tribune with a February spreadsheet of providers in DuPage County, which showed the outsize holdings at Petlak’s practice. But it has yet to comply with a Tribune open records request for all providers’ daily inventory.

The Illinois Department of Public Health said it’s setting up National Guard sites to help address vaccination disparities. “No matter if you’re in the city, the suburbs or the rural community, we want to get everyone access to vaccines,” Friend, of IDPH, told state senators.

But for now, some National Guard sites are helping to showcase the differences.

Records show that more than 1,400 Chicago-area residents signed up for vaccination appointments at a mass site in Springfield hours away — an outcome that surprised local health officials.

“I assumed there were opportunities for vaccinations in their hometowns,” Sangamon County’s health director, Gail O’Neill, told the Tribune.

It’s difficult to tell how many people are traveling far for shots — the state declined to release data to the Tribune that could be used to measure it. But health officials across the state say the trend is widespread, and it could be affecting vaccination rates.

Perhaps the biggest example is Chicago.

City data shows that some Chicagoans have gotten shots outside the city, but it’s been more common for outsiders to cross into Chicago. As a result, the city has seen a net loss of about 160,000 doses, a Tribune analysis found. If all of those shots had been injected into Chicago residents, the city’s vaccination rate would be higher than those in the suburban counties.

The issue came to a head at the United Center vaccination site, which was initially open to any senior citizen. Then Chicago officials complained that nonresidents were elbowing out Chicagoans who needed the shots more (such as people living in West Englewood). So the site stopped offering slots to seniors from the collar counties, which in turn prompted complaints that those seniors were unfairly stuck with fewer options.

Cook County also has limited slots at its mass vaccination sites to people who live or work in that health department’s suburban jurisdiction, even though the state says clinics run with the National Guard should be open to any qualifying Illinoisan.

At Monday’s state Senate hearing, IDPH’s director, Dr. Ngozi Ezike, acknowledged the tensions statewide. She said local officials are eager to boost their jurisdiction’s vaccination numbers, which are measured by where people live, not where they get shots.

“In my ideal world it would be, ‘We’re all one state,’” she said. “But I understand people putting that (residency restriction) on because everybody’s responding to their locally elected officials who want to see their numbers (improve).”

Hani Mahmassani, a Northwestern University professor studying the logistical challenges of vaccination, said residents should be prepared for more mass confusion as new waves of residents start searching for shots.

How long that will last is a matter of debate. Ezike told state senators she suspects that, with federal officials pushing out more supplies, doses will “really ramp up” in the coming weeks, growing “really significantly” by April. That optimism has led the state to set April 12 as the date anyone 16 and up can begin signing up for shots, at least outside Chicago.

“In the meantime, we have this constant undersupply,” Ezike told state senators. “It will be for a few more weeks, if we could just hang on.”

Of course, Chicagoans can still opt to make a 10-hour round-trip drive to Quincy. A Friday afternoon check of that facility’s website found that most appointment slots there still were open, waiting for sign-ups.

Tribune reporters Madeline Buckley and Dan Petrella contributed.