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Why is Delta the most contagious variant?

The Wuhan strain of SARS-CoV-2, from which the pandemic originated, has been used to develop currently available vaccines. Since its appearance, several variants have emerged, such as the Alpha (UK), Beta (South African), Gamma (Brazilian) and Delta (Indian). Amongst the molecules that make up SARS-CoV-2, the spike (S) is a key protein. It interacts on the surface with the ACE2 cellular receptor and facilitates the fusion of viral and cellular membranes to enable entry into the cell. The Delta variant is now dominant, replacing the Alpha variant that was itself twice as transmissible as the original strain. American researchers (at the Harvard Medical School) have studied the S protein to try and understand the mechanisms that lead to its high transmissibility.

The researchers began by studying the fusion of viral and cellular membranes that the S protein enables. They also studied the structure of the S protein in the Delta variant using electron microscopy. They showed firstly that the S protein of the Delta variant may facilitate entry even into cells expressing low levels of the ACE2 cellular receptor. They then observed that Delta makes it possible for the virus to attach to the cell more efficiently, and enables faster membrane fusion than in other variants. However, in the S protein RBD (Receptor Binding Domain), which binds to the ACE2 cellular receptor to enable the virus’ penetration, the mutations present in the Delta variant do not modify affinity for the ACE2 receptor.

Delta is the most contagious variant. But will it be replaced by a new variant with other properties allowing it to enter cells even more effectively?

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