Moderna has about 5.9 million doses ready for shipment set to begin over the weekend, according to Operation Warp Speed, the government's vaccine development program.

WASHINGTON — The U.S. added a second COVID-19 vaccine to its arsenal Friday, boosting efforts to beat back an outbreak so dire that the nation is regularly recording more than 3,000 deaths a day.

Much-needed doses are set to arrive Monday after the Food and Drug Administration authorized an emergency rollout of the vaccine developed by Moderna Inc. and the National Institutes of Health.

The move marks the world's first authorization for Moderna's shots. The vaccine is very similar to one from Pfizer Inc. and Germany's BioNTech that's now being dispensed to millions of health care workers and nursing home residents as the biggest vaccination drive in U.S. history starts to ramp up.

The two work "better than we almost dared to hope," NIH Director Dr. Francis Collins told The Associated Press. "Science is working here, science has done something amazing."

Early results of large, still unfinished studies show both vaccines appear safe and strongly protective although Moderna's is easier to handle since it doesn't need to be stored at ultra-frozen temperatures.

The nation is scrambling to expand vaccinations as rapidly as Moderna and Pfizer can churn out doses. Moderna's is for people 18 and older, Pfizer's starts at age 16.

"What we want to always remember is one size does not fit all. We want to have options," said Dr. Paul Duprex of the University of Pittsburgh.

Moderna has about 5.9 million doses ready for shipment set to begin over the weekend, according to Operation Warp Speed, the government's vaccine development program. Injections of health workers and nursing home residents continue next week, before other essential workers and vulnerable groups are allowed to get in line.

Both Moderna's and Pfizer-BioNTech's shots are so-called mRNA vaccines, made with a groundbreaking new technology. They don't contain any coronavirus – meaning they cannot cause infection. Instead, they use a piece of genetic code that trains the immune system to recognize the spike protein on the surface of the virus, ready to attack if the real thing comes along.

Their development less than a year after the coronavirus first emerged set a speed record, but Collins stressed that shouldn't worry people. The speed was due to billions in company and government investments paired with years of earlier scientific research, not any cut corners.

"The rigor of the analysis of these vaccines is unprecedented," Collins said. "We're not done with this but hope is on the way, and the hope comes from this scientific brain trust that has pulled out all the stops."

Experts are hoping the two vaccines together will "break the back of the pandemic" when combined with masks and other precautions, said Dr. Arnold Monto of the University of Michigan, who chaired an advisory committee that publicly debated the shots' evidence ahead of FDA's decisions.

This is a breaking news report.