After the virus binds to ACE2, the next step is to fuse with the cell, a process that begins when enzymes from the host cell cut the spike at two different sites, a process known as cleavage. This kick starts the fusion machinery. If binding is like the key fitting in the lock, cleavage is like the key turning the deadbolt. “Without cuts at both sites, the virus can’t get into cells,” says Vineet Menachery, a virologist at The University of Texas Medical Branch.
One of the mutations present in Delta actually occurs in one of these cleavage sites, and a new study that has not yet been peer reviewed shows that this mutation does enhance cleavage. And Menachery, who was not involved in the study, says he has replicated those results in his lab. “So it’s a little bit easier for the virus to be activated,” he says.
Whether that improves transmissibility isn’t yet known, but it could. When scientists delete these cleavage sites, the virus becomes less transmissible and less pathogenic, Menachery says. So it stands to reason that changes that facilitate cleavage would increase transmissibility.
It’s also possible that Delta’s ability to evade the body’s immune response helps fuel transmission. Immune evasion means more cells become infected and produce more virus, which then potentially makes it easier for person carrying that virus to infect someone else.
But vaccines still work
The good news is that vaccination provides strong protection against Delta. A new study from Public Health England shows that the Pfizer-BioNTech vaccine was 88% effective in preventing symptomatic disease 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 one dose of either vaccine, however, was much lower— just 33%.
In any case, in the US and UK, just around 42% of the population is fully vaccinated. In India, where the virus surged fueled in part by the rapid spread of Delta, just 3.3% of the population has achieved full vaccination.
At the press briefing, Fauci urged those who have not been vaccinated to get their first shot and reminded those who are partially vaccinated not to skip their second dose. The Biden Administration hopes to have 70% of the population at least partially vaccinated by the Fourth of July. In the UK, Delta quickly replaced Alpha to become the dominant strain, and cases are now on the rise. “We cannot let that happen in the United States,” Fauci said.