RT Journal Article SR Electronic T1 Ineffective neutralization of the SARS-CoV-2 Mu variant by convalescent and vaccine sera JF bioRxiv FD Cold Spring Harbor Laboratory SP 2021.09.06.459005 DO 10.1101/2021.09.06.459005 A1 Uriu, Keiya A1 Kimura, Izumi A1 Shirakawa, Kotaro A1 Takaori-Kondo, Akifumi A1 Nakada, Taka-aki A1 Kaneda, Atsushi A1 The Genotype to Phenotype Japan (G2P-Japan) Consortium A1 Nakagawa, So A1 Sato, Kei YR 2021 UL http://biorxiv.org/content/early/2021/09/07/2021.09.06.459005.abstract AB On August 30, 2021, the WHO classified the SARS-CoV-2 Mu variant (B.1.621 lineage) as a new variant of interest. The WHO defines “comparative assessment of virus characteristics and public health risks” as primary action in response to the emergence of new SARS-CoV-2 variants. Here, we demonstrate that the Mu variant is highly resistant to sera from COVID-19 convalescents and BNT162b2-vaccinated individuals. Direct comparison of different SARS-CoV-2 spike proteins revealed that Mu spike is more resistant to serum-mediated neutralization than all other currently recognized variants of interest (VOI) and concern (VOC). This includes the Beta variant (B.1.351) that has been suggested to represent the most resistant variant to convalescent and vaccinated sera to date (e.g., Collier et al, Nature, 2021; Wang et al, Nature, 2021). Since breakthrough infection by newly emerging variants is a major concern during the current COVID-19 pandemic (Bergwerk et al., NEJM, 2021), we believe that our findings are of significant public health interest. Our results will help to better assess the risk posed by the Mu variant for vaccinated, previously infected and naïve populations.Competing Interest StatementThe authors have declared no competing interest.