The move affects an estimated 3,000 to 3,500 people, according to Delaware's Fire Prevention Commission.
The men and women on the front lines are seeing high COVID-19 call volume and are concerned about when they'll be fully protected from the virus.
"To be told a few days before you're supposed to get your second dose, it's a little disconcerting," says Robert Pappa, a Talleyville firefighter.
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Originally scheduled for Wednesday, Pappa, says his second dose vaccine appointment has been canceled. Thousands of other first responders in Delaware got the same alert.
"A lot of us are experiencing a lot of burnout because of everything going on, so we saw this vaccine as kind of a relief from that," says Pappa.
Notice of the change went out to firehouses throughout Delaware on Friday, which affects about 50 to 55 people in the Christiana Fire Department.
"We weren't given any specific reasons, but it's an indication there are some issues with supply," says Asst. Chief Dennis Godek of the Christiana Fire Dept.
The Delaware Department of Public Health says just that. Supply is low and new guidance from the CDC says a second dose can be just effective six weeks (42 days) after the first dose, so the DPH has paused first responder second dose vaccine events.
Dr. Margot Savoy from Temple's Lewis Katz School of Medicine explains the new guidance.
"You can actually wait up to 42 days without there being a problem getting the second dose. So don't get too excited or too worried if you are not able to get the dose exactly on time," Savoy said.
With the clock counting down, Pappa has 16 days until he must get the second dose.
"It could be a week late, it could be a few days late, as of right now it's up in the air, we have no idea when we would be able to get our second dose," said Pappa.
Dr. Anthony Fauci says the first dose is somewhat effective, but 95% immunity comes seven to 10 days after the second dose. That can't come soon enough for the people on the front lines.