PHOENIX — Pima and Maricopa counties will divide up Arizona’s first allocation of COVID-19 vaccines, while rural areas will have to wait a while.

Steve Elliott, spokesman for the state Department of Health Services, said it’s not a question of deciding that urban residents are entitled to a higher priority. He said it’s a matter of practicality.

The first vaccine to arrive will be from Pfizer, and that vaccine has to be kept at minus-70 degrees Celsius (about 94 degrees below zero Fahrenheit), requiring access to special equipment. The shipments come in minimum doses of 975.

“The big lots and handling requirements of the Pfizer vaccine make it better-suited to places where many people can be vaccinated in one place in a finite period,’’ Elliott said.

The first shipment is due to arrive next week.

Maricopa County will get 46,800 doses by the end of December, with 11,700 for Pima, a figure that about parallels their relative difference in population.

And the rest of the state?

Elliott said residents not in Maricopa or Pima counties will be in line for the vaccine developed by Moderna, which needs to be kept only at minus-20 Celsius (about minus-4 Fahrenheit), more like a regular freezer. Moderna will ship out the vaccine in lots of a minimum of 100.

“So rural counties will see vaccine in the following week,’’ Elliott said.

Overall, as vaccines become available, Arizona should get about 2% of the production based on its share of the national population.

State health officials report that 593 health-care providers have completed the approval process and are certified to start administering the vaccine when it arrives. More than 1,200 more are in the pipeline.

Every skilled nursing facility in Arizona has opted to participate in a program by the Centers for Disease Control and Prevention to provide vaccinations to all residents and staff.

Both of the vaccines require a second dose, within 21 or 28 days, to be fully effective.

The vaccines are expected to be shipped with the complete supplies for administering them, including syringes, personal protective equipment and cards to remind people when to come back for the second dose. The state also is using its own tracking system to ensure that people get both doses.

Under the state plan, the top priority for immunization will be health-care workers, particularly those working directly with patients. Also in the first group will be home health aides, nursing assistants and medical assistants.

After that come residents of skilled nursing facilities and independent and assisted living centers.

Future deliveries will serve those in a second tier, including adults with high-risk medical conditions living in shelters. Teachers also are in this group along with other school workers like bus drivers and cafeteria staff, as are police and emergency response workers.

Health officials say it could be summer — or early fall — before there are sufficient vaccines for everyone in Arizona who wants to be inoculated.

Still unknown is how long the effects last and whether people who are vaccinated are forever immune to the coronavirus or might need booster shots or even regular updates like the seasonal flu.