The Delta variants (there’s actually more than one in the same viral family) have about 15 different mutations compared with the original virus. Two of these, L452R and E484Q, are mutations to the spike protein that were first flagged as problematic in other variants because they appear to help the virus escape the antibodies we make to fight it.
It has another mutation away from its binding site that’s also getting researchers’ attention — P681R.
This mutation appears to enhance the “springiness” of the parts of the virus that dock onto our cells, said Alexander Greninger, MD, PhD, assistant director of the UW Medicine Clinical Virology Laboratory at the University of Washington in Seattle. So it’s more likely to be in the right position to infect our cells if we come into contact with it.
Another theory is that P681R may also enhance the virus’s ability to fuse cells together into clumps that have several different nuclei. These balls of fused cells are called syncytia.
“So it turns into a big factory for making viruses,” said Kamran Kadkhoda, PhD, medical director of immunopathology at the Cleveland Clinic in Ohio.
This capability is not unique to Delta or even to the new coronavirus. Earlier versions and other viruses can do the same thing, but according to a recent paper in Nature, the syncytia that Delta creates are larger than the ones created by previous variants.
Scientists aren’t sure what these supersized syncytia mean, exactly, but they have some theories. They may help the virus copy itself more quickly, so a person’s viral load builds up quickly. That may enhance the ability of the virus to transmit from person to person.
And at least one recent study from China supports this idea. That study, which was posted ahead of peer review on the website Virological.org, tracked 167 people infected with Delta back to a single index case.
China has used extensive contact tracing to identify people that may have been exposed to the virus and sequester them quickly to tamp down its spread. Once a person is isolated or quarantined, they are tested daily with gold-standard PCR testing to determine whether or not they were infected.
Researchers compared the characteristics of Delta cases with those of people infected in 2020 with previous versions of the virus.
This study found that people infected by Delta tested positive more quickly than their predecessors did. In 2020, it took an average of 6 days for someone to test positive after an exposure. With Delta, it took an average of about 4 days.
When people tested positive, they had more than 1,000 times more virus in their bodies, suggesting that the Delta variant has a higher growth rate in the body.
This gives Delta a big advantage. According to Angie Rasmussen, PhD, a virologist at the Vaccine and Infectious Disease Organization at the University of Saskatchewan in Canada, who posted a thread about the study on Twitter, if people are shedding 1,000 times more virus, it is much more likely that close contacts will be exposed to enough of it to become infected themselves.
And if they’re shedding earlier in the course of their infections, the virus has more opportunity to spread.
This may help explain why Delta is so much more contagious.
Beyond transmission, Delta’s ability to form syncytia may have two other important consequences. It may help the virus hide from our immune system, and it may make the virus more damaging to the body.
Commonly, when a virus infects a cell, it will corrupt the cell’s protein-making machinery to crank out more copies of itself. When the cell dies, these new copies are released into the plasma outside the cell where they can float over and infect new cells. It’s in this extracellular space where a virus can also be attacked by the neutralizing antibodies our immune system makes to fight it off.
“Antibodies don’t penetrate inside the cell. If these viruses are going from one cell to another by just fusing to each other, antibodies become less useful,” Dr. Kadkhoda said.