A 50 microgram booster dose—the authorized amount, which is half the dose used for primary immunization—saw a 37-fold increase in neutralizing antibodies, the company said in a statement Monday. The company also tested a 100 microgram dose, which increased antibody levels 83-fold compared with the primary two-dose course.

The results add to a growing body of evidence that three shots will be needed to neutralize the fast-spreading omicron. Pfizer Inc. and BioNTech SE said earlier this month that a third shot of their vaccine restored protection to a level similar to the initial two-dose regimen against the original virus. 

The data are “reassuring,” Moderna Chief Executive Officer Stephane Bancel said in a statement. “To respond to this highly transmissible variant, Moderna will continue to rapidly advance an omicron-specific booster candidate into clinical testing in case it becomes necessary in the future.”

The shares rose 6.5% in premarket trading in New York. 

“The actual fold increase is only valuable if it’s compared with other vaccines,” said Sam Fazeli, a Bloomberg Intelligence analyst. ”These levels should increase protection against infection but the key question is how long do they last.”

The Moderna data are based on lab tests using blood sera from 20 booster recipients with each dose, with antibody levels measured on day 29 post-boost, the company said. Moderna said it plans to submit the results for online publication.

Moderna is testing different booster candidates against a range of variants in mid and late-stage trials. The biotech said it plans to start testing its Omicron-specific vaccine in humans early next year. It has also been testing the safety and tolerability of the higher 100 microgram booster dose. 

The higher dose level was generally well tolerated, with adverse events at a similar frequency to those experienced after the primary two-dose course. There were slightly more frequent side effects from the 100 microgram third dose than the 50 microgram booster, the company said.