In addition to becoming more transmissible, some variants have also acquired the ability to dodge some of our antibodies. Antibodies, which can prevent the virus from entering our cells, are engineered to latch onto specific molecules on the surface of the virus, snapping into place like puzzle pieces. But genetic mutations in the virus can change the shape of those binding sites.
“If you change that shape, you can make it impossible for an antibody to do its job,” said Marion Pepper, an immunologist at the University of Washington School of Medicine.
Delta appears to evade some antibodies, but there are other variants, particularly Beta, that are even better at dodging these defenses. For now, Delta is so infectious that it has managed to outcompete, and thus limit the spread of, these stealthier variants.
But as more people acquire antibodies against the virus, mutations that allow the virus to slip past these antibodies will become even more advantageous. “The landscape of selection has changed,” said Jessica Metcalf, an evolutionary biologist at Princeton University. “From the point of view of the virus, it’s no longer, ‘I just bop around, and there’s a free host.’”
The good news is that there are many different kinds of antibodies, and a variant with a few new mutations is unlikely to escape them all, experts said.
“The immune system has also evolved to have plenty of tricks up its sleeve to counteract the evolution of the virus,” Dr. Pepper said. “Knowing that there is this complex level of diversity in the immune system allows me to sleep better at night.”
Certain T cells, for instance, destroy virus-infected cells, helping to reduce the severity of disease. Together, our assortment of T cells can recognize at least 30 to 40 different pieces
of SARS-CoV-2, researchers have found.
“It’s a lot harder to evade T cell responses than antibody responses,” said Dr. Celine Gounder an infectious disease specialist at the New York University Grossman School of Medicine.
And then there are B cells, which generate our army of antibodies. Even after we clear the infection, the body keeps churning out B cells for a while, deliberately introducing small genetic mutations. The result is an enormously diverse collection of B cells producing an array of antibodies, some of which might be a good match for the next variant that comes along.
“They’re actually a library of guesses that the immune system makes about what variants might look like in the future,” said Shane Crotty, a virologist at the La Jolla Institute for Immunology.
So far, studies suggest that our antibody, T cell and B cell responses are all working as expected when it comes to SARS-CoV-2. “This virus is mostly playing by immunological rules we understand,” Dr. Crotty said.