Moderna's Covid-19 vaccine is 94 percent effective at preventing symptomatic illness and appears to prevent the spread of the virus as well, according to documents released Tuesday.
The findings set the Moderna vaccine up for emergency use authorization by the Food and Drug Administration by the end of this week, meaning Americans could soon have two highly effective Covid-19 vaccines, after the first shots of Pfizer-BioNTech's vaccine were given to health care workers on Monday.
The high efficacy of the Moderna vaccine was noted after two doses given 28 days apart. This is about the same level of effectiveness as the Pfizer vaccine.
But there is also evidence suggesting that just one dose of Moderna's may stop the virus's spread. A second document published on the FDA website Tuesday shows asymptomatic infection was reduced by 63 percent after the first shot.
More data are needed to fully understand that finding.
"We're holding our breath," said Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical Center in Nashville, Tennessee.
"If we also reduce transmission — make people much less apt to be contagious — as well as protecting them from serious disease, that would be a double bonus," Schaffner, who is not involved with the Moderna vaccine research, said.
Even those who have been closely involved with Moderna's Phase 3 clinical trials advise caution when interpreting the preliminary results that suggest the vaccine curbs transmission.
"Keep in mind, these are small numbers," said Dr. Hana El Sahly, principal investigator for the Moderna studies at Baylor College of Medicine. "Regardless, it's a trend in the right direction."
It is expected that regulators will require two doses of the vaccine for maximum protection. Until it's clear that any vaccine effectively prevents spread of the virus, tried and true methods of prevention remain critical.
"We need to be using all of the interventions we have in our arsenal right now," said Jessica Atwell, an infectious disease epidemiologist at the Johns Hopkins Center for Immunization Research. "We know that hand washing and masking and physical distancing can reduce transmission from one person to another."
A committee independent of the FDA, the Vaccines and Related Biological Products Advisory Committee, will meet Thursday to decide whether to recommend that the agency greenlight the Moderna vaccine. The meeting will largely mirror the one from last Thursday, when the panel of experts ultimately recommended that the FDA authorize Pfizer's vaccine.
The most common side effects associated with the Moderna vaccine were fatigue, headache and muscle pain. Rarely, nausea and facial swelling were also reported, according to the FDA documents.
El Sahly acknowledged that "with the second dose, there's an increase in the symptoms of fatigue, headache, and fever, compared to the first dose," but added that those symptoms were manageable.
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Overall, the data showed people over age 64 had fewer side effects than younger adults.
The vaccines from both drugmakers work the same way, using tiny snippets of genetic code, called mRNA, to train the body to fight off the coronavirus.
It is widely anticipated that the Moderna vaccine's path to authorization will be similar to Pfizer's. An authorization could come as soon as Friday. Moderna will ask for authorization for adults over age 18.
Indeed, officials with Operation Warp Speed are already planning for widespread distribution of the Moderna vaccine.
On Monday, Army Gen. Gustave Perna, the chief operating officer of Operation Warp Speed, said that, assuming the FDA authorizes the Moderna vaccine, nearly 6 million doses will be shipped to 3,285 locations across the country.