Distribution of the second COVID-19 vaccine authorized in the U.S. started on Sunday as initial shipments left a factory in the Memphis area.

Injections of the vaccine developed by Moderna Inc. and the National Institutes of Health are expected to begin on Monday, some three days after the Food and Drug Administration authorized their emergency rollout.

The development comes as the number of COVID-19 infections in the U.S. surpassed the 17.66 million mark with a death toll exceeding 316,000, according to data from the Johns Hopkins University.

Public health experts say the Moderna Inc vaccine and others still in the pipeline are the only way to stop the virus.

Another vaccine from Pfizer Inc. and Germany's BioNTech began distribution a week ago, with injections beginning a day later.

According to the AP, both the new Moderna vaccine and the Pfizer-BioNTech shot require two doses several weeks apart. The second dose must be from the same company as the first. Both vaccines appeared safe and strongly protective in large, still unfinished studies.