A surge in Covid-19 cases following the emergence of the highly transmissible delta variant has sparked fresh health concerns and raised questions about the safety and pace of reopening, even in places where vaccination rates are high such as the U.K. Sam Fazeli, a Bloomberg Opinion contributor who covers the pharmaceutical industry for Bloomberg Intelligence, answers questions about the variant and its potential threat. The conversation has been edited and condensed.
The delta variant has caused an uptick in Covid cases, especially among the unvaccinated but also among those who have received their shots. Isn’t it true, though, that vaccines offer good protection, even against this variant?
Well, it’s complicated by definitions. We know that vaccines were never 100% effective against an infection even before delta arrived. In the trials of Pfizer Inc.-BioNTech SE and Moderna Inc. vaccines, for example, we saw efficacy over 90%, meaning very few people tested positive for the virus. What was critical, though, was that vaccinated people in these trials were 100% protected against severe disease or hospitalization. The delta variant, as we will discuss below, has some characteristics that give it a better chance of causing an infection, which means it escapes the initial shield provided by antibodies. But vaccinated people are still at a far lower risk of developing severe disease, or if they do, their symptoms resolve more quickly. However, let’s not be under any illusions—people will still die from Covid-19 even if they are vaccinated, but in far lower numbers than if they were not.
What makes delta such a threat? How does it work?
The delta variant has developed certain mutations that make it more pernicious than its relatives. It has become better at infecting cells, partly by virtue of being able to somewhat evade antibodies in either previously infected or vaccinated people. Once inside cells, it is better at replicating. Evidence for this is confirmed by much higher amount of virus (viral load) in the nasal swabs of people who are infected with delta compared with those seen in the first wave of the pandemic. That may come from a process called syncytium formation, in which infected cells fuse with normal neighbors. The process helps the virus hide from the immune system and replicate faster. The delta variant seems to make bigger clusters, which helps it create more potentially infectious copies. A higher viral load also means that an infected person may exhale more virus particles, giving it a better chance to find its next victim. All of this leads to a much higher “fitness.”
What are the risks of contracting a “breakthrough” infection?
Again, we need to be very careful here. Breakthrough infections are not at all surprising, given that we knew the vaccines were never 100% effective against an infection. And the variants are eroding their efficacy in this setting. The virus infects people through the lining of their respiratory tracts, otherwise known as the mucosal membranes. These areas may not have as many vaccine antibodies, which gives the virus a small foothold. But an infection alone may not be a reason for concern in vaccinated individuals.
How severe are the breakthrough cases we’re seeing? How concerning is it that even populations with high vaccination rates, such as Israel and the U.K., are seeing more of these?
Fortunately, we are not seeing breakthrough disease, i.e., the vaccines’ effectiveness against severe disease and hospitalization is still very high regardless of the individual variant. And remember that if vaccinated people do get an infection, their immune system will respond and actually cause some of the symptoms that we are used to with other infections, such as headache, a stuffy nose, and muscle/joint ache. Obviously people are not the same—some will have a much stronger antibody response to a vaccine than others and their immune system response to an infection will also be different, meaning different cold/flu-like symptoms. And with time a decline in antibody levels in some people who started out with a weaker immune response could permit a mild infection.