The emergence of SARS-CoV-2 variants threatens efforts to control the COVID-19 pandemic. At present, the global spread of the Delta (B. 1.617. 2) variant is responsible for a rapid increase in COVID-19 cases and hospitalizations in many countries. The variant evades neutralizing antibodies and is believed to be more transmissible and pathogenic [1-4]. Neutralizing antibodies are produced upon vaccination or infection; for the latter, it is known that levels correlate with the duration and severity of clinical symptoms [5]. Antibody evasion is caused by mutations in the viral spike protein, several of which are located in the receptorbinding domain, which is the key target of the neutralizing antibody response.

Recently, sublineages of the Delta (B. 1.617. 2) variant have emerged, termed Delta Plus [6], which are purported to be more transmissible. These viruses harbor the K417N mutation (Fig. 1 A, B), which is also found in the Beta (B. 1.351) variant and is associated with neutralization resistance [7]. Furthermore, another sublineage of the Delta variant was observed in Vietnam and might contribute to a recent surge in cases. This variant (provisionally termed Delta-V) is reported to contain mutations found in the S protein of the Alpha (B. 1.1. 7) variant, and it was initially proposed to be a hybrid virus. Indeed, Delta variant sequences from Vietnam and many other countries with deletions at positions H67, V70, and/or Y144 (Fig. 1 A, B), also found in spike of the Alpha (B. 1.1. 7) variant, have been deposited in the GISAID (Global Initiative on Sharing All Influenza Data) database. Thus, sublineages of the Delta variant have arisen that might have altered biological properties and may present an increased threat to human health.