There are two Moderna boosters that have been announced, the mRNA-1273.211 and mRNA-1273.214, ones. The .211 candidate is based on the Beta variant spike proteins the trial of which has already been completed, whereas the trial of the .214 candidate that is based on Omicron mutations is expected to complete by or in early June 2022.

From https://investors.modernatx.com/news/news-details/2022/Moder... :

  Moderna has multiple bivalent booster candidates that have been evaluated to date, which include mRNA-1273.211 (9 spike protein mutations, based on the Beta variant), and mRNA-1273.214 (32 spike protein mutations, based on the Omicron variant).  mRNA-1273.211 includes four mutations and mRNA-1273.214 includes 32 mutations present in the Omicron variant of concern.

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  A booster dose of mRNA-1273.211 demonstrated superiority against the ancestral SARS-CoV-2 and the Beta, Delta and Omicron variants one month after the booster dose and superiority against the ancestral SARS-CoV-2, Beta and Omicron 6 months compared to the booster dose of mRNA-1273. There was a 2.20-fold (95% CI: 1.74, 2.79) and 2.15-fold (95% CI: 1.66, 2.78) increase in the neutralizing antibody titers against Omicron with the mRNA-1273.211 booster dose compared to the mRNA-1273 booster dose at 1 month and 6 months, respectively.