Delta serves as a textbook example. Its genetic code not only carries some of the successful mutations of earlier variants but also the ability to spread twice as fast, churning out many more progeny.
Data indicate that delta is 40%-60% more transmissible than alpha and almost twice as transmissible as the original Wuhan strain of SARS-CoV-2. Furthermore, it grows at higher levels inside the throat and lungs than did earlier versions of the virus.
With only 49.3% of the U.S. population fully vaccinated, this trait is proving key to the delta’s continued evolution and spread. That is why it has quickly become California’s dominant strain, accounting for 84.4% of all new sequenced cases, up from 53% in June.
“It’s sobering,” said virologist Shane Crotty of the Vaccine Discovery Division at La Jolla Institute for Immunology. Variants “alpha and delta are six months apart from each other. Each managed to become much more infectious.
“That definitely makes it less certain about what the virus can do in the future,” he said.
Scientists are racing to understand what specific mutations are causing transmission to soar. But it could also involve changes to an enzyme called RNA polymerase, which is responsible for copying genetic code as the virus reproduces, said Ernst.
“With influenza, we know that there are variants of RNA polymerase that will make more viruses, because the viral polymerase works better,” he said. Or perhaps it’s some other part of the virus that excels at its job.
Changes to two other traits — virulence and vaccine evasion — also could transform the pandemic.
The most risky changes involve the virus’ spike protein, which lets the virus latch onto human cells and gain entry, causing infection.
“The concern is that each mutation might make it a little bit more able to bind,” said infectious disease expert Dr. Julie Parsonnet, professor of medicine at Stanford University.