WASHINGTON — Johnson & Johnson’s single-dose vaccine offers strong protection against severe COVID-19, according to an analysis released Wednesday by U.S. regulators that sets the stage for a final decision on a new and easier-to-use shot to help tame the pandemic.

The shot could offer the nation a third vaccine option and help speed vaccinations by requiring just one dose instead of two. Food and Drug Administration scientists confirmed that overall the vaccine is about 66% effective at preventing moderate to severe COVID-19, and about 85% effective against the most serious illness. The agency also said J&J’s shot is safe.

The analysis is one step in the FDA’s evaluation. On Friday, the agency’s independent advisers will debate if the evidence is strong enough to recommend the shot. With that advice, the FDA is expected to make a decision within days.

The COVID-19 death toll in the U.S. topped 500,000 this week, and the vaccination drive has been slower than hoped, hampered by logistical and weather delays. So far, about 44.5 million Americans have received at least one dose of vaccine made by Pfizer or Moderna, and nearly 20 million of them have received the second dose required for full protection.