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DENVER (KDVR) — Saturday, more COVID-19 vaccination sites opened up in the Denver metro, as millions more Coloradans will soon be eligible for the COVID-19 vaccine.

Dr. Michelle Barron of UCHealth says not many people have missed vaccine appointments within their medical system.

“I think most people are pretty motivated to try and have it done,” Barron said. “I would encourage everyone to try and stick to a schedule; It’s complicated to reschedule.”

Barron says if you do miss an appointment for a second dose, you have a 42-day window to reschedule.

“Feel a little bit of comfort that you still have 42 days from the appointment to get the next shot,” Barron said. “If you go beyond that window, you’ve got to start over.”

She estimates the first dose of the Moderna or the Pfizer vaccine is a big help, but the second dose is critical for our bodies to ward off the disease.

“Both of them, if you average them out, is about 40 to 60 percent [efficacy at first dose],” Barron said.  “The booster idea is telling your immune system you’ve seen this before and if it comes at you, you’re prepared.”

Barron says, as long as a patient receives their second dose within the 42-day time frame, there is no reason to be concerned about reduced efficacy.

“There’s no way your immune system would behave any differently than if you got it on time,” Barron said.

If your clinic cancels an appointment, medical experts say the clinic should be reaching out to you to reschedule your appointment.