CAL20.C SARS-CoV-2 variant skyrocketing in California evades host immune response The effect of different neutralizing antibodies, both vaccine-elicited and from convalescent sera, suggests new variants are emerging that can evade the human immune response. The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) binds to host cells via its spike protein, which has two parts, the S1 and S2 subunits. The S1 subunit has the receptor-binding domain (RBD) and the N-terminal domain (NTD). The RBD binds to the host angiotensin-converting enzyme 2 (ACE2) to infect host cells. Neutralizing antibodies are produced against both the RBD and NTD by infected and vaccinated individuals. Some RBD-specific monoclonal antibodies are currently under clinical trials or approved for use to treat COVID-19 patients.