Efficacy of Antiviral Agents against the SARS-CoV-2 Omicron Subvariant BA.2

To the Editor: The omicron (B.1.1.529) variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which is responsible for coronavirus disease 2019 (Covid-19), has spread rapidly around the world and has already become the predominant variant circulating in many countries. As of February 2022, omicron variants have been divided into four distinct sublineages: BA.1, BA.1. 1, BA.2, and BA.3. 1 Most circulating omicron variants belong to sublineage BA.1; however, in Denmark, India, and the Philippines, the sublineage BA.2 is now becoming dominant.2 As compared with the Wuhan/Hu-1/2019 reference strain, the sublineage BA.2 of the omicron variant has 16 amino acid substitutions in the receptor-binding domain of the spike (S) protein of SARS-CoV-2,2 which is the primary target for monoclonal antibody–based therapy. The BA.2 and BA.1 variants share 12 of these 16 substitutions; however, BA.2 has four substitutions in the receptor-binding domain (i.e., S371F, T376A, D405N, and R408S) that differ from those in BA.1. These findings suggest that there may be differences in the effectiveness of monoclonal antibodies against these different omicron sublineages. Accordingly, we examined the neutralizing ability of therapeutic monoclonal antibodies that have been approved by the Food and Drug Administration, individually and in combination, against the omicron BA.2 subvariant hCoV-19/ Japan/UT-NCD1288-2N/2022 (omicron/BA.2; NCD1288), which was isolated from a traveler who arrived in Japan from India. Whole-genome sequencing analysis of the NCD1288 virus stock confirmed that it had the 16 substitutions that are characteristic of the omicron variant in the receptor-binding domain of the S protein, as compared with the Wuhan/Hu-1/2019 reference strain (Table S1 in the Supplementary Appendix, available with the full text of this letter at NEJM.org). A live-virus focus reduction neutralization test (FRNT) showed that both LY-CoV016 (marketed as etesevimab) and LY-CoV555 (marketed as bamlanivimab), individually and in combination, lost neutralizing activity against omicron/BA.2 (NCD1288) (Table 1). These findings are similar to our previous findings with omicron/BA.1 (hCoV-19/Japan/NC928-2N/2021; NC928)3 and omicron/BA.1. 1 (hCoV-19/Japan/NC929-1N/2021; NC929). 4 BA.1. 1, a subvariant of BA.1, has the R346K mutation in the S protein (Table S2). However, REGN10987 (marketed as imdevimab), which was previously shown to lose neutralizing activity against omicron/BA.1 (NC928) and omicron/BA.1. 1 (NC929),3,4 had neutralizing activity against omicron/BA.2 (NCD1288). In addition, the combination of REGN10987 and REGN10933 (marketed as casirivimab) also inhibited omicron/BA.2 but did not inhibit omicron/BA.1 or omicron/BA.1. 1. However, the FRNT50 (the titer of monoclonal antibodies required for a 50% reduction in the number of infectious foci) value of this combination therapy was higher by a factor of 43.0 to 143.6 for omicron/BA.2 than for an ancestral strain — SARS-CoV-2/UT-NC0021T/Human/2020/Tokyo (NC002) — and other variants of concern (i.e., the alpha [B.1.1.7], beta [B.1.351], gamma [P.1], and delta [B.1.617.2] variants). REGN10933, COV2-2196 (marketed as tixagevimab), and COV2-2130 (marketed as cilgavimab) neutralized omicron/BA.2. The COV2-2196–COV22130 combination inhibited omicron/BA.2 with a low FRNT50 value (14.48 ng per milliliter); however, the FRNT50 values of this combination were higher by a factor of 1.4 to 8.1 for omicron/BA.2 than for the ancestral strain and other variants of concern. S309 (the precursor of sotrovimab), which has been shown to have lower neutralizing activity against omicron/BA.1 and omicron/BA.1. 1 than against the ancestral strain and other variants of concern,3,4 had even less neutralizing activity against omicron/BA.2 in our study. The FRNT50 value of this monoclonal antibody was higher by a factor of 12.2 to 49.7 for omicron/BA.2 than