If researchers were predicting which coronavirus variant would take over the world, the Delta variant would not have been their first guess. But since its first appearance in India in December 2020, the highly contagious variant has become the predominant strain of the virus, accounting for more than 90 percent of new COVID cases in the U.S.
Delta’s emergence has caused a number of countries to reinstate travel and mask restrictions that had been loosened as vaccination rates rose. Although the vaccines appear mostly effective against Delta, the sheer number of cases increase the likelihood that it could cause “breakthrough infections” in vaccinated people. And it’s still unclear whether it results in more severe disease than the previously circulating strains.
What is clear, however, is that Delta has a strong evolutionary advantage over earlier strains. “Its rate of increase is unlike any other in the history of this pandemic,” says Vaughn Cooper, an evolutionary biologist at the University of Pittsburgh. Now, he and others are trying to figure out why this particular variant, which carries a suite of different mutations, has been so successful.
The virus’s rapid spread may in part result from a fast replication rate. One recent study found that people infected with Delta had, on average, around 1,000 times more virus in their bodies—known as viral load—than those infected with the original strain, allowing them to infect more people quickly. The variant’s advantage seems to stem from a combination of mutations in the spike protein: the part of the novel coronavirus that binds to ACE2 receptors on the surface of cells and allows the virus to infect them.
Scientists have also wondered whether Delta—in addition to its increased transmissibility—is able to escape the human immune system. It lacks a mutation called E484K, which helps a number of other variants partially avoid being neutralized by antibodies. But lab studies suggested that that a Delta mutation called L452R was even better at performing the same function.