The virus continues to spread rapidly in many parts of the world, even as segments of the population have gained some degree of immunity as a result of having been infected or vaccinated.
Scientists say that combination—high rates of viral transmission and a partially immunized population—encourages the emergence of variants that are potentially more transmissible or more lethal. More transmission means more opportunities for the virus to evolve, they say.
“If everyone has immunity, then you have pretty much no virus circulating and the virus can’t adapt,” University of Bern molecular epidemiologist Emma Hodcroft said, adding that if no one within a population has immunity, then there’s no pressure on the virus to evolve. “That middle part, where you have a partially vaccinated population, or a partially immune population with lots of virus circulating, that’s kind of your danger point,” she said.
The key to minimizing these problems, they say, lies with social distancing and other measures to reduce contagion as well as with ramping up vaccination efforts, which have lagged behind in many places.
“The slower you are at both of these things, the bigger risk you’re taking that you’re going to have the emergence of more variants,” said Richard Lessells, an infectious-disease specialist.