The World Health Organization designated omicron as a variant of concern in November 2021. It possesses at least 33 mutations in its spike (S) protein than the early SARS-CoV-2 strains. 

Interestingly, 15 of the 29 substitutions are in the receptor-binding domain of the S protein, which is the primary target for monoclonal antibody-based therapy, which further suggests that the monoclonal antibodies, approved by the Food and Drug Administration (FDA) may show lowered effectiveness against the omicron variant.

The omicron variant has mutations in both the RNA-dependent RNA polymerase (RdRp) and the main protease of SARS-CoV-2, which are targeted by antiviral drugs like RdRp inhibitors (remdesivir and molnupiravir) and the main protease inhibitor PF-07304814, which also provokes concern regarding the decreased effectiveness of these drugs against omicron. Thus, three different antiviral compounds (i.e., remdesivir, molnupiravir, and PF-07304814) were tested for their efficacy against omicron. The in vitro 50% inhibitory concentration (IC50) values were determined against NC928, NC002, HP127, HP01542, TY7-503, and UW5250 for each compound. Omicron showed similar susceptibilities to the three compounds with those of the early strain, which suggest that all three of these compounds may show efficacy for treating patients infected with the omicron variant.

SOURCE- New England Journal of Medicine, 2022. 10.1056/NEJMc2119407. https://www.nejm.org/doi/full/10.1056/NEJMc2119407