With a brief prick of a needle, Pennsylvania’s journey to inoculate the state against COVID-19 began Monday.

“Here we go,” University of Pittsburgh Medical Center nurse Tami Minnier said as she injected the state’s first vaccine dose just after 11 a.m. into the shoulder of a nurse practitioner.

“We clearly have weeks and months to go,” said Minnier, a registered nurse and the hospital’s chief quality officer, “but we are so excited to take this first step today.”

The recipient, Charmaine Pykosh, receiving her inoculation of the Pfizer and BioNTech vaccine before a room of photographers, gave a thumbs up and indicated she was smiling beneath her mask.

“They told me to smile with my eyes,” said the acute care nurse practitioner, who has worked at UPMC for 30 years.

She was among five frontline workers at the UPMC Children’s Hospital to receive the first doses of the long-awaited vaccine. Some of Philadelphia’s health care workers are to get their first shots in the coming days, and Einstein Hospital reported Monday it had received its first doses.

Pennsylvania Health Secretary Rachel Levine called Monday “a new chapter in this fight.”

Pennsylvania is set to receive 97,500 doses this week via direct shipments to 87 hospitals across the commonwealth, she said. An additional 13,650 doses are going to Philadelphia, which is managing distribution in the city through its health department independently of the state.

”It’s tremendously exciting,” Levine said.

» READ MORE: Philadelphia hospitals prepare to give COVID-19 vaccines this week. Who will go first?

The vaccine is being shipped across the country Monday in a mass vaccination campaign to end a pandemic that has killed almost 300,000 Americans, infected more than 16 million, and paralyzed the nation since March. The doses will go to 145 sites Monday, 425 on Tuesday, and 66 on Wednesday, Army Gen. Gustave F. Perna, chief operating officer for Operation Warp Speed, said in a news briefing Saturday.

Having a vaccine approved and distributed in less than a year is, “positively amazing,” Graham Snyder, medical director of UPMC’s infection prevention and hospital epidemiology, said in a news conference after Monday’s inoculations.

The Food and Drug Administration approved the vaccine manufactured by Pfizer and BioNTech for emergency use just Friday night. Another vaccine from Moderna is expected to receive FDA emergency use authorization this week.

» READ MORE: Pfizer COVID-19 vaccine looks highly effective, even for high-risk patients, new data indicate

The state’s supply of vaccine will arrive in 100 trays, each carrying 975 doses.

UPMC Children’s Hospital received the health system’s first doses Monday, and the first five recipients were chosen from across the health system. ,The health system expects to receive 9,756 through this week. Pykosh’s coworkers took an informal vote to select her to get the first shot after being notified last Thursday the vaccine would soon arrive. Sunday night, she said, “my heart was pounding.”

“It was exciting, relieving, and as I said before,” Pykosh said, “I’m honored to do this.”

The Pittsburgh health system has 90,000 employees, and it will take “a couple months” for them all to be vaccinated, Snyder said.

There remains much uncertainty around when and how the vaccine will be distributed after this first phase, which is focused largely on health care workers. Levine reiterated, “mitigation now is more important than ever during these winter months.”

Pennsylvania will receive additional shipments of the Pfizer vaccine in the weeks to come, but Levine said she did not have a solid estimate of how many doses would be included in those shipments. Front line health care workers and residents of long-term care facilities will be among those who could get their first doses of the vaccine by month’s end. If Moderna’s vaccine gets emergency use authorization by the FDA, it could also be shipped out across the commonwealth next week, Levine added.

The general public likely won’t be vaccinated before the spring, she said.

Also unknown, Snyder said, was how long the vaccine would provide immunity from the coronavirus.

» READ MORE: COVID-19 survivors may develop some immunity, but still should get the vaccine. Here’s why.

Minnier compared Monday’s shots to a vaccine for polio, a virus now virtually extinct that once killed 2% to 5% of all children infected and up to 15% to 30% of adults.

“This moment is happening just a few miles from another historical time that happened here in Pittsburgh over 65 years ago,” Minnier said. “April 12, 1955 Doctor Jonas Salk [developer of the polio vaccine] took some of these same steps.”