A huge U.S. study of another COVID-19 vaccine candidate got underway Monday as states continue to roll out scarce supplies of the first shots to a nation anxiously awaiting relief from the catastrophic outbreak.

Public health experts say more options in addition to the two vaccines now being dispensed — one made by Pfizer and its German partner BioNTech, the other by Moderna — are critical to amassing enough shots for the country and the world.

The candidate made by Novavax Inc. is the fifth to reach final-stage testing in the United States. Some 30,000 volunteers are needed to prove if the shot — a different kind than its Pfizer and Moderna competitors — really works and is safe.

“If you want to have enough vaccine to vaccinate all the people in the U.S. who you'd like to vaccinate — up to 85% or more of the population — you're going to need more than two companies,” Dr. Anthony Fauci, the top U.S. infectious disease expert, told The Associated Press on Monday.