KITCHENER -- The Region of Waterloo continues to open more COVID-19 vaccine appointments amid high demand for booster shots. 

Adults 18 and older were eligible for a third dose as of Monday morning.

As of 9:50 a.m., the region said all clinics were fully booked. However more appointments were added at multiple locations throughout the day.

The pop-up vaccine bus was at the Kitchener Public Library on Monday morning and hit capacity before opening at 9 a.m.​

The vaccine bus clinic was a walk-in site accepting residents in need of their first, second or third dose of a COVID-19 vaccine.

“Vaccination buses will be placed in priority communities where residents are facing the greatest access barriers to vaccination, leading to increased COVID risk," the region said in an emailed statement to CTV Kitchener. "Currently, vaccination buses are intended for members of these communities."

Herbert Reiko was first in line at around 6:45 a.m.

"I just took a chance," he said. "I get up early anyways, I knew it was going to be cold, but I didn't realize for two hours how cold you can really get."

Reiko says he had to guess where he thought the bus was going to be at the library and then waited by that spot. He was soon joined by Kris McQueen, who was second in line.

"He looked lonely out there so I thought I'd join him," he said. "I was going to wait out here for a couple hours anyways whether I came at 7 a.m. or 9 a.m."

At 8:50 a.m., estimated 500 people were in the line, which stretched around the library and into the underground parking lot, when most were told they only had capicity for 50-60.

"I didn't realize there was such a limit," said Johnny Moreau, who arrived at 8:15 a.m but was told to leave. "Seems kind of strange for what was supposed to be such a big vaccination push for everyone to get one. It's too bad. I'll come back when I can get one as soon as possible."

A Region of Waterloo spokesperson later confirmed with CTV News that they only had abut 50 doses available at the mobile clinic.

"We sincerely apologize for the frustration and disappointment this caused," the spokesperson said in a statement. "We are continuously adding clinic dates and appointments to our vaccination clinic schedules as we have the health and human resources capacity to do so."

For those that did get on to the bus like Genvieve Digle, the process took about five minutes.

"Just sit down, fill out a questionnaire, which arm, here's your jab, here's your sticker, and you're good to go," she said.

Digle adds that, while it might be discouraging for some, to keep looking for booster availability.

"Keep trying," she said. "The first time I looked for a shot, the first one I could find was at the end of January, and then I kept looking and I found one for mid-January, then I found one for Jan. 7, and then I saw this one, and now it's done.

"If you have to line up for a few hours, line up for a few hours and then it's done."

The mobile vaccination clinic was scheduled to run from 9 a.m. to 10:30 a.m.

The province reported that 125,000 people had booked through the portal as of 10 a.m.

Region of Waterloo Public Health uses a different vaccine appointment portal than the province.

In Huron Perth, the online booking had to close temporarily due to demand.

“We are continuously adding more clinics and more appointments as capacity allows,” a statement from the health unit said. “Even clinics that are currently indicated as full may have more appointments added.”

Wellington-Dufferin-Guelph Public Health had issues with the booking site on Monday morning, but the issue was resolved by the afternoon.

“We are going to have some hiccups along the way, naturally, given the scale and pace at which we are working,” Associate Medical Officer of Health Dr. Matthew Tenenbaum said. “We are asking everyone to be patient both in terms of getting through to us, as well as in terms of getting their actual shot.”