Once the virus has bound to ACE2, the next step is to fuse with the cell, a process that begins when the host cell’s enzymes cut the tip at two different sites, a process known as of cleavage. This kick starts the fusion machinery. If the binding is like the key that goes into the lock, the cleavage is like the key that turns the deadbolt. “Without cuts at both sites, the virus cannot enter cells,” says Vineet Menachery, a virologist in the medical branch of the University of Texas.
One of the mutations present in Delta actually occurs in one of these cleavage sites, and a new study that has not yet been peer reviewed shows that this mutation enhances cleavage. And Menachery, who was not in the study, says he’s replicated those results in his lab. “So it’s a little easier to activate the virus,” he says.
It is not yet known if this improves transmissibility, but it could. When scientists remove these cleavage sites, the virus becomes less transmissible and less pathogenic, Menachery explains. It stands to reason, then, that changes that facilitate cleavage would increase transmissibility.
It is also possible that Delta’s ability to evade the body’s immune response may help fuel transmission. Immune escape means more cells are infected and produce more virus, which can then make it easier for a person with that virus to infect someone else.
But vaccines still work
The good news is that vaccination offers strong protection against Delta. A new study from Public Health England shows that the Pfizer-BioNTech vaccine was 88% effective in preventing symptomatic illnesses due to Delta in fully vaccinated people. The AstraZeneca vaccine provided slightly less protection. Two shots were 60% effective against the variant. The effectiveness of a dose of either vaccine, however, was much lower – only 33%.
Either way, in the US and UK, around 42% of the population is fully vaccinated. In India, where the virus has surged, fueled in part by the rapid spread of Delta, only 3.3% of the population has achieved full vaccination.
During the press conference, Fauci urged those who have not been vaccinated to get the first shot and reminded those who are partially vaccinated not to skip their second dose. The Biden administration hopes that 70% of the population will be at least partially vaccinated by July 4. In the UK, Delta quickly replaced Alpha to become the dominant strain, and cases are now on the rise. “We can’t let this happen in the United States,” Fauci said.