Comparison of Plaque Size, Thermal Stability, and Replication Rate among SARS-CoV-2 Variants of Concern

ABSTRACT

SARS-CoV-2, like other RNA viruses, has a propensity for genetic evolution owing to the low fidelity of its viral polymerase. This evolution results in the emergence of novel variants with different characteristics than their ancestral strain. Several recent reports have described a series of novel SARS-CoV-2 variants. Some of these have been identified as variants of concern (VOCs), including alpha (B.1.1.7, Clade GRY), beta (B.1.351, Clade GH), gamma (P.1, Clade GR), and delta (B.1.617.2, Clade G). VOCs are likely to have some effect on transmissibility, antibody evasion, and changes in therapeutic or vaccine effectiveness. However, the physiological and virological understanding of these variants remains poor. We demonstrated that these four VOCs exhibited differences in plaque size, thermal stability at physiological temperature, and replication rates. The mean plaque size of beta was the largest, followed by those of gamma, delta, and alpha. Thermal stability, evaluated by measuring infectivity and half-life after prolonged incubation at physiological temperature, was correlated with plaque size in all variants except alpha. However, despite its relatively high thermal stability, alphas small plaque size resulted in lower replication rates and fewer progeny viruses. Our findings may inform further virological studies of SARS-CoV-2 variant characteristics, VOCs, and variants of interest. These studies are important for the effective management of the COVID-19 pandemic. IMPORTANCEThe global pandemic caused by SARS-CoV-2 continues to persist, due in part to mutations that have resulted in the emergence of different variants. Many of these variants have become more virulent and infectious than their ancestral strain, resulting in an ever-increasing spread. However, our virological understanding of these variants remains poor. Here, we directly compared the plaque size, stability, and replication kinetics of four SARS-CoV-2 variants of concern following prolonged incubation at physiological temperatures. Our observations may help to characterize each variant in terms of their interactions with host factors and responses to environmental conditions. We also believe that our evaluations will improve understanding of the emergence of new variants and contribute to controlling their spread.