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 activates the fusing machine If the bond is like the key that fits in the lock, the split is like the key that turns the lock. “Without cuts at both sites, the virus cannot enter the 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One of the mutations present in Delta actually occurs in one of these cleavage sites, and a new study that has not yet been peer-reviewed shows that this mutation increases cleavage. And Menachery, who was not involved in the study, says he replicated these results in his lab. “So it’s a little easier for the virus to be activated,” he says.
Whether it improves transmissibility is not yet known, but it could. When scientists delete these cleavage sites, the virus becomes less transmissible and less pathogenic, Menachery says. Therefore there is reason that changes that facilitate splitting will increase transmissibility.
It is also possible that Delta’s ability to elude the body’s immune response helps fuel transmission. Immune evasion means that more cells become infected and produce more viruses, which potentially facilitates the 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 individuals. The AstraZeneca vaccine provided slightly less protection. Two shots were 60% effective against the variant. The effectiveness of a dose of both vaccines, however, was much lower – only 33%.
However, in the United States and the United Kingdom, only about 42% of the population is completely vaccinated. In India, where the virus has developed fueled in part by the rapid spread of Delta, only 3.3% of the population has achieved full vaccination.
At the press briefing, Fauci encouraged those who were not vaccinated to receive 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 has quickly replaced Alpha to become the dominant strain, and homes are now growing. “We can’t let that happen in the United States,” Fauci said.