The following article originally appeared in FiercePharma and was written by Arlene Weintraub

With Moderna and Pfizer both reporting sky-high response rates to their COVID-19 vaccines, the pressure is on federal health officials to ensure a rapid—but smooth—rollout. Wednesday, they unveiled a detailed timeline that provides some clues about when most Americans can expect to be vaccinated.

High-priority populations such as healthcare workers and nursing home residents could obtain COVID-19 vaccines in December, Department of Health and Human Services Secretary Alex Azar said in a press conference. Anthony Fauci, M.D., director of the National Institute of Allergy and Infectious Diseases, predicted in a meeting with USA Today’s editorial board that everyday Americans could be vaccinated as early as April.

Azar and Fauci were clearly encouraged by the flood of good news from Pfizer, its vaccine partner BioNTech and Moderna. It started Monday, when Moderna said its mRNA COVID-19 vaccine was 94.5% effective in a phase 3 trial. Then Pfizer updated its previous report of 90% efficacy, saying yesterday that its vaccine was 95% effective in a phase 3 trial. Pfizer expects to seek emergency use authorization from the FDA presently.

The key to distributing the vaccines to older Americans quickly is a distribution deal the government struck with Walgreens and CVS last month. Last week, more agreements were formed with pharmacy chains and independent pharmacies, Azar said.

“We have seen tremendous uptake of that option already,” Azar said yesterday. “Ninety-nine percent of skilled nursing facilities across the country have signed up, and 100% of facilities in 20 states are signed up.” Forty million vaccine doses will be available in the U.S. by the end of this year, he added.

Azar has said in several media appearances that all healthcare workers and first responders should be able to access COVID-19 vaccines by the end of January.

But meeting that goal, and providing widespread vaccination by April, will hinge on Pfizer and Moderna avoiding delays that are all too common in the FDA review process. Moncef Slaoui, Ph.D., co-leader of the U.S. government’s Operation Warp Speed program, warned in an interview with Fierce Pharma last month that “an approval is going to take several weeks from the moment one says, ‘OK, the vaccine is efficacious.'”

Warp Speed has been fostering the development of six additional COVID-19 vaccines, but it’s clear Pfizer and Moderna’s mRNA vaccines will be the first out of the gate. Moderna expects to ship 20 million doses for use in the U.S. by the end of this year and up to 1 billion worldwide next year. Pfizer is gearing up to ship 50 million doses this year.

The distribution of COVID-19 shots could raise a whole new set of hurdles. Both mRNA front-runners require ultra-cold storage and stringent temperature control to remain potent. That has sent airlines, shipping companies and cargo carriers on a scramble to prepare for the massive vaccination rollout.