Johnson & Johnson's single-dose vaccine offers strong protection against severe COVID-19, according to an analysis by US regulators that sets the stage for a final decision on a new and easier-to-use shot to help tame the pandemic.
The long-anticipated 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 on Wednesday confirmed that overall the vaccine was about 66 per cent effective at preventing moderate to severe COVID-19 and about 85 per cent effective against the most serious illness. The agency also said J&J's shot was safe.
The analysis is just 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 final decision within days.
The COVID-19 death toll in the US 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.
Tests showed the Pfizer and Moderna vaccines were 95 per cent effective at protection against symptomatic COVID-19.
Dr Paul Offit, a vaccine expert at Children's Hospital of Philadelphia, is part of the FDA advisory panel that will scrutinise the J&J data on Friday and cautions that none of the vaccines have been directly compared.
Still, he is encouraged that one dose of the J&J vaccine appears as good at preventing serious illness as its two-dose competitors.
"This is a vaccine to prevent you from going to the hospital and dying at a level that's certainly comparable" to the Pfizer and Moderna vaccines, he said.
J&J tested its single-dose option in 44,000 adults in the US, Latin America and South Africa. The FDA analysis cautioned that it was not clear how well the vaccine worked against different variants.
But J&J previously announced the vaccine worked better in the US - 72 per cent effective against moderate to severe COVID-19, compared with 66 per cent in Latin America and 57 per cent in South Africa.
Australian Associated Press