• Moderna said that its currently authorized booster can "boost neutralizing antibody levels 37-fold higher than pre-boost levels," which it described as reassuring.
• However, it also said that a double dose of the booster shot — 100 micrograms, rather than the approved 50 micrograms — was significantly more effective.

Moderna announced Monday that a third dose of its mRNA vaccine against Covid-19 appears to provide significant protection against the omicron variant.

The company said that its authorized booster can "boost neutralizing antibody levels 37-fold higher than pre-boost levels," which it described as reassuring.

However, it also said that a double dose of the booster shot — 100 micrograms, rather than the approved 50 micrograms — was significantly more effective. A 100 microgram booster dose increased neutralizing antibody levels "approximately 83-fold," according to preliminary data, it said in a release.

The 100 microgram dose of its booster shot was "generally safe and well tolerated," Moderna said, though "there was a trend toward slightly more frequent adverse reactions following the 100 [microgram] booster dose relative to the authorized 50 [microgram] dose." 

Without a booster, the company's vaccine, called mRNA-1273, was been found to be far less effective against the fast-spreading omicron variant, generating low neutralizing antibodies.

The data have not yet been peer reviewed.

The company said it will also continue to develop an omicron-specific booster candidate which should enter clinical trials in early 2022.

The decision to focus on its current Covid-19 booster, rather than the variant-specific vaccine, was because of both the speed of omicron's spread and its confidence in its current offering, the company said.

"Moving forward, given the strength of the mRNA-1273 and the speed at which the Omicron variant is spreading, Moderna's first line of defense against Omicron will be a booster dose of mRNA-1273," the company said in its statement.

"Given the long-term threat demonstrated by Omicron's immune escape, Moderna will also continue to develop an Omicron-specific variant vaccine (mRNA-1273.529) that it expects to advance into clinical trials in early 2022 and will evaluate including Omicron in its multivalent booster program."

The news comes as the World Health Organization warns omicron cases can double every 1.5 to 3 days in areas of local spread. The U.K. recorded its highest daily case levels since the start of the pandemic in the last week, and the Netherlands imposed a strict full lockdown on Sunday extending past the new year.

Moderna's vaccine had been facing some issues after both the company and the Centers for Disease Control and Prevention acknowledged that it was linked to a higher risk of heart inflammation in young men, known as myocarditis.

Moderna maintains that the protection the shot offers against severe disease, hospitalization and death outweighs the extremely rare risk of myocarditis.

Despite this, France in November became the latest European country to recommend that adults younger than 30 receive Pfizer's Covid-19 vaccine instead of the Moderna's. Denmark, Iceland, Finland and Sweden have also restricted the Moderna shot for young people.