Abstract
The coronavirus disease 2019 (COVID-19) pandemic has highlighted the importance of developing novel vaccines. An ideal vaccine should trigger an intense immune reaction without causing significant side effects. In this study we found that substitution of tryptophan located in the cores of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) protein structures with certain smaller amino acids resulted in variants with melting temperatures of 33-37 °C. An enzyme activity assay indicated that the proteolytic activity of the main proteinase (3CL(pro)) decreased sharply when the environmental temperature exceeded the melting temperature, implying that other protein variants may lose most of their functions under the same conditions. This finding suggests that a virus variant containing engineered proteins with melting temperatures of 33-37 °C may only be functional in the upper respiratory tract where the temperature is about 33 °C, but will be unable to invade internal organs, which maintain temperatures above 37 °C, thus making it possible to construct temperature-sensitive attenuated vaccines.