CHICAGO (CBS) — In one of the biggest steps in getting more people in Illinois vaccinated against COVID-19, the state moves to Phase 2 of its vaccination program on Monday.

Under Phase 2, everyone 16 and older will be eligible for the vaccine – except in Chicago, where the date is a week later.

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The state has already opened more than 150,000 appointments.

But there is always concern about supply and demand. On Sunday night, CBS 2’s Steven Graves looked into one major hurdle for teenagers and their parents.

The 16- and 17-year-olds can only get the Pfizer vaccine – Moderna and Johnson & Johnson are not approved for that age range. Some challenges are already arising as local counties prepare for a big influx of interest.

You’ve met 16-year-old Avi Rubin on CBS 2 before, as he has been helping hundreds of people book COVID-19 vaccine doses.

“Now that I have the opportunity to get the vaccine, I’m really excited,” said Rubin, a Highland Park High School student.

But even though Rubin has cracked the code to find shots, getting one himself is a challenge. Again, those like him who are under 18 are only eligible for the Pfizer vaccine.

“You’re kind of competing with all these adults also who are looking for a vaccine,” Rubin said.

And given that eligibility is opening Monday for everyone 16 and over in Illinois – with no restrictions about who qualifies – that means a lot more people.

Chicago is not included in Phase 2 yet, but city leaders have told residents to go to the suburbs if possible.

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State and county mass vaccination sites, hospital systems, and pharmacies remain the best places to go.

Rubin says Walgreens provides the best shot at Pfizer for minors.

“It’s really difficult,” he said. “There’s nowhere that’s reserving it for teenagers, really.”

We checked with Cook, DuPage, Will, and Kane Counties, which have sites open to state residents. All say it is a first-come first-served basis for teenagers, but most have large Pfizer supply.

Many county websites, like that for Kane County, have specific dosage information.

“We do expect to see a good spike in demand over the next couple of weeks, but I think we’re countering that with increased capacity and availability to make appointments,” said Michael Isaacson of the Kane County Health Department.

In Cook County – where young people recently fueled a COVID case spike – health leaders say vaccine expansion should help.

“We want to make sure no one is left behind. I think it remains to be seen what direction we are going to go,” said Dr. Kiran Joshi of Cook County Health. “We are seeing that there’s been a little bit of a flattening, and so I’m hoping that things stay calm.”

And for teens like Rubin, a vaccine offers a long-awaited return to normalcy – even creating excitement over possibly stepping foot back in school.

“If enough people go vaccinated, I would very happily go back in person,” he said.

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Parents or guardians should be present for minors to get the shot, and they should also have a form of ID.