In New Jersey, the Moorestown Mall megasite opens Monday after a soft opening on Friday, and organizers urged people to make appointments online using Virtua's online portal.
"You're going to have to be patient. This is volumes we have never seen in our systems before. The volumes are shutting us down," said John Mattsinger, executive vice president and COO for Virtua Health during a media tour of the site on Friday.
State officials say the number of eligible people in New Jersey far exceeds the number of doses available, so appointments may be hard to come by.
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"We've been in a quarantine state for a very long time and I think it's time for us to go outside and be humans," said Loc Phan of Pennsauken.
But there are also people who are hesitant about the new vaccine.
"I'm going to wait a little while to see what the turnaround is, see if there's any problems," said Astor DeGuzman.
A newly-vaccinated healthcare worker said she hopes there will be enough for those who want it.
"I'm hoping that at some point it will be distributed to everyone," said Camika Braswell, a nurse from Burlington Township.
In Philadelphia, organizers at the Pennsylvania Convention Center vaccination site say they could be vaccinating more people if they had more doses. City health officials say while they are still vaccinating healthcare workers in phase 1a, they are making plans on when to begin phase 1b, adding that they're constrained by receiving 20,000 doses per week.
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On Sunday morning, Director of the National Institute of Allergy and Infectious Diseases Dr. Anthony Fauci addressed what he called "a misunderstanding" about the national stockpile, and how long the federal government would hold back second doses of the vaccine.
"When it became clear that the cadence of the, of the flow of doses, was really going to be consistent and reliable, the decision was made instead of just giving enough for the first dose and holding back for the second dose, that as soon as they got the doses available, they would give it," said Fauci.
Fauci also said the vaccines developed by AstraZeneca and Johnson & Johnson could be weeks away from being evaluated by the Food and Drug Administration.