US experts will meet Thursday to decide whether to recommend approval of Moderna's Covid-19 vaccine, potentially paving the way for rollout early next week.
The meeting comes after the Pfizer-BioNTech vaccine received an emergency use authorization and the first three million doses were distributed in the world's worst-hit country this week.
Thursday's meeting will be live-streamed to the public and will end with a vote by the two dozen independent scientists and industry representatives.
Should the panelists vote in favor, as is widely expected, the Food and Drug Administration is expected to issue its green light soon after, which could make the US the first country to approve the Moderna vaccine.
The small Massachusetts-based biotech firm teamed up with scientists from the US National Institutes of Health to produce the product and began work in January just days after Chinese authorities shared the SARS-CoV-2 virus genome.
Moderna has received more than $2.5 billion from the US government for its efforts and has promised six million doses in the first wave.
A clinical trial of 30,400 people found it was 94.1 percent effective in preventing Covid-19 compared to a placebo, performing slightly better in younger adults compared to the elderly.
An FDA review of all available data found there were "no specific safety concerns identified that would preclude the issuance of an EUA (emergency use authorization)."
Moncef Slaoui, the chief advisor of Operation Warp Speed, told reporters on Wednesday that he was impressed by the level of protection against disease the vaccine provided 10 to 14 days after even the first of two doses.
"The second thing that's really interesting is the apparent protection against infection," which was shown to be 60-65 percent.
An outstanding concern has been how well vaccines will prevent people from becoming asymptomatically infected, and Slaoui said he expected the efficacy to become even higher after the second dose, given 28 days later.
"I think that would be very important from a public health perspective," he said.
The most common side effects associated with the drug, called mRNA-1273, were injection site pain in roughly 90 percent of cases; fatigue in 70 percent, headache in 60 percent, muscle pain in 60 percent, joint pain in 45 percent and chills in 45 percent.
Few of these effects were classed as "severe" and, when they were, that tended to occur more in the young than in the old.
Lymphadenopathy, or swollen lymph nodes, occurred in 1.1 percent of the vaccine group against 0.6 percent of the placebo group.
There was an imbalance in allergic reactions, which occurred in 1.5 percent of the vaccinated population compared to 1.1 percent of the non-vaccinated.
But none were classed as severe or anaphylactic.
The frequency of life-threatening events was low, around one percent in each group, with no suggestion the vaccine was the cause.
To date, there have been three reports of Bell's palsy -- a facial paralysis condition, most often temporary -- in the vaccine group and one in the placebo group.
The Pfizer trial saw four people get Bell's palsy in the vaccine group and none in the placebo group.
The FDA said there was insufficient information to determine that either vaccine was the cause.
Both the Pfizer and Moderna vaccines use mRNA (messenger ribonucleic acid) molecules to cause human cells to express a surface molecule of the new coronavirus.
This simulates an infection and trains the immune system to be ready in case it encounters the real virus.
The mRNA is encased inside fatty particles.
Both companies used a slightly different formulation resulting in different cold-storage requirements: -70 degrees Celsius (-94 degrees Fahrenheit) for Pfizer; -20 degrees Celsius (-4 Fahrenheit) for Moderna.
Moderna has applied for approval in over-18s, while Pfizer's approval is for over-16s.
The US is hoping to immunize 20 million people in December, with health care workers and long-term care residents at the front of the line.