Moderna started a clinical trial to study the safety and effectiveness of a booster shot that specifically targets the omicron Covid variant, the company announced Wednesday.

The first participant in the phase two trial has already received a dose of the omicron-specific booster shot, according to the company.

Moderna expects to enroll about 600 adult participants ages 18 and over split equally between two groups. Participants in the first group will have previously received two-doses of Moderna’s original vaccine, and participants in the second group will have previously received the two-dose vaccine and the currently authorized booster shot.

Participants in both groups will receive a single dose of the omicron specific booster.

Moderna also published data on how its 50 microgram booster shot is holding up against omicron. Six months after the third shot, the antibodies that block omicron infection declined, but they still remained detectable in all participants. The data indicates that while the booster weakens over time, it still provides some protection against the variant.

The neutralizing antibodies triggered by Moderna’s existing booster declined faster when pitted against omicron, 6.3 fold, compared with a decline of about 2.3 fold against the original strain of the virus over the same time period.

CEO Stephane Bancel said Moderna is reassured the neutralizing antibodies against omicron remain detectable after half a year.

“Nonetheless, given the long-term threat demonstrated by Omicron’s immune escape, we are advancing our Omicron-specific variant vaccine booster candidate,” Bancel said.

Bancel previously said he expected the efficacy of booster doses would likely declined over time.

Pfizer and BioNTech said they started testing their omicron-specific vaccine on people earlier this week, enrolling 1,420 participants ages 18 to 55. The companies expect to have the shots ready by March.