Delta variant (B.1.617.2) of SARS-CoV-2: current understanding of infection, transmission, immune escape, and mutational landscape

Abstract

The Delta variant is one of the alarming variants of the SARS-CoV-2 virus that have been immensely detrimental and a significant cause of the prolonged pandemic (B.1.617.2). During the SARS-CoV-2 pandemic from December 2020 to October 2021, the Delta variant showed global dominance, and afterwards, the Omicron variant showed global dominance. Delta shows high infectivity rate which accounted for nearly 70% of the cases after December 2020. This review discusses the additional attributes that make the Delta variant so infectious and transmissible. The study also focuses on the significant mutations, namely the L452R and T478K present on the receptor-binding domain of spike (S)-glycoprotein, which confers specific alterations to the Delta variant. Considerably, we have also highlighted other notable factors such as the immune escape, infectivity and re-infectivity, vaccine escape, Ro number, S-glycoprotein stability, cleavage pattern, and its binding affinity with the host cell receptor protein. We have also emphasized clinical manifestations, symptomatology, morbidity, and mortality for the Delta variant compared with other significant SARS-CoV-2 variants. This review will help the researchers to get an elucidative view of the Delta variant to adopt some practical strategies to minimize the escalating spread of the SARS-CoV-2 Delta variant.

Keywords: Delta variant; Distribution; Immune escape; Infectivity; SARS-CoV-2.

Fig. 1

The timeline represents the series of events of the SARS-CoV-2 Delta variant first report, naming, and transmission

Fig. 2

The figure shows the distribution of Delta variants in different countries. The figure illustrates the Delta variant distribution throughout the globe, such as the USA, Europe, Latin America, South America, South Africa, different Asian countries, and Australia. The Delta variant was first found in India and spread throughout the world. The figure was developed by the Nextstrain server (Nextstrain 2021)

Fig. 3

The figure depicts continent-wise sequence counts of the Delta variant. The figure represents the 7-day rolling average of sequence counts (continent-wise). The figure was developed by the CoV-lineages server (O'Toole 2021)

Fig. 4

The figure illustrates the continent-wise frequency of sequences of the B.1.617.2 lineage. The figure represents the seven-day rolling average. The figure was developed by the CoV -lineages server (O'Toole 2021)

Fig. 5

Continent-wise estimate of the absolute number of cases of Delta variants. A Asia, B Europe, C Africa, D North America, E South America. The CoV-spectrum server developed the figure. Data were obtained from the GISAID server, which was analyzed through the web application (Elbe and Buckland‐Merrett 2017)

Fig. 6

Schematic diagram and 3D surface structural model shows different significant mutations position in the S-glycoprotein of Delta variant of SARS-CoV-2. A Schematic diagram of significant mutations of Delta variant in S-protein. B Mutations of Delta variant S-protein surface structure in front view. C Mutations of Delta variant S-protein surface structure 90° axial rotated view. D Mutations of Delta variant S-protein surface structure 90° crosswise turned view