CHICAGO (CBS) — Coronavirus vaccine appointment sign-ups at the United Center will soon open to even more people than just seniors.

But as CBS 2’s Steven Graves reported Saturday, some glitches are causing concern over if the sign-up website can handle another influx.

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Late Saturday, crews working here on site to start vaccinations next week. But some wonder if that registration website needs a little work before that can happen.

That is because Friday and Saturday, people were noticing emails with cancellations or rescheduling. It comes after they were one of the tens of thousands who booked slots last Thursday on the ZocDoc site. The emails called the cancellations “an unfortunate technical issue.”

The greatest fear is that eligible seniors signing up now will show up here next week and not be able to get the shot at all.

“To just literally watch the cancellation emails roll in is so incredibly frustrating,” said Victoria Cardenas.

Messages also went out from the City of Chicago about a rescheduled slot, and then a follow-up that says there was a mistake.

“OK, we’ve booked all these people for these appointments,” said John Berkebile. “Like, is my phone just going to be ringing the days of that they’re like, ‘Hey, I showed up the time that you guys booked and they, like, turned me away.’”

Berkebile and Cardenas are considered Vaccine Angels – part of a huge Facebook group that helped dozens of seniors who don’t have technology sign up last Thursday.

“And it’s scary to think that they could possibly be canceled, and especially with the influx that’s going to happen tomorrow,” Cardenas said.

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Starting at 4 p.m. Sunday, the appointment pool extends to those 16 and older with medical conditions – only amplifying concern over if more issues could come.

“And if you magnify that by thousands of people, just by sheer probability, there’s bound to be confusion and overlap – not that I think that the people who set up the booking system didn’t try their absolute best,” said Susie Prather of Vaccine Angels.

It is unclear what is causing the website glitches. The Vaccine Angels helpers think efforts to sign up multiple people at once might be the issue.

“If a couple is sharing an email – like an elderly couple – and it’s just like an account for the both of them, at least one if not both of their appointments will get canceled because they’re using that shared email account,” Cardenas said.

But the frustration is that there are no clear answers.

“We just really feel that there are certain barriers that make it very hard for seniors,” Cardenas said.

We’re waiting on responses from ZocDoc and the City to clear up confusion over what exactly went wrong.

We’re working to get answers from Chicago city officials about what’s going on. But officials are encouraging seniors to call the UC hotline which now has expanded its capacity.

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The number for that hotline is (312) 746-4835.