Serum Neutralizing Activity of mRNA-1273 Against the SARS-CoV-2 B.1.1.529 (Omicron) Variant: A Preliminary Report

The emergence of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) B.1.1.529 (Omicron) variant has led to growing concerns of increased transmissibility and escape of both natural and vaccine-induced immunity. In this analysis, sera from adult participants in a phase 2 clinical study ( <ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="clintrialgov" xlink:href="NCT04405076">NCT04405076</ext-link> ) were tested for neutralizing activity against B.1.1.529 after a 2-dose (100 µg) mRNA-1273 primary vaccination series and after a 50-µg mRNA-1273 booster dose. Results from this preliminary analysis show that 1 month after completing the primary series, mRNA-1273-elicited serum neutralization of B.1.1.529 was below the lower limit of quantification; however, neutralization was observed at 2 weeks after the mRNA-1273 booster dose, although at a reduced level relative to wild-type SARS-CoV-2 (D614G) and lower than that observed against D614G at 1 month after the primary series.