Why No One Is Sure If Delta Is Deadlier


The variants are spreading faster, but they don’t necessarily have incentive to kill more often.


The coronavirus is on a serious self-improvement kick. Since infiltrating the human population, SARS-CoV-2 has splintered into hundreds of lineages, with some seeding new, fast-spreading variants. A more infectious version first overtook the OG coronavirus last spring, before giving way to the ultra-transmissible Alpha (B.1.1.7) variant. Now Delta (B.1.617.2), potentially the most contagious contender to date, is poised to usurp the global throne.

Alphabetically, chronologically, the virus is getting better and better at its primary objective: infecting us. And experts suspect that it may be a while yet before the pathogen’s contagious potential truly maxes out. “A virus is always going to try and increase its transmissibility if it can,” Jemma Geoghegan, an evolutionary virologist at the University of Otago, told me.

Other aspects of the virus’s unfolding bildungsroman, however, are much harder to forecast, or even get an initial read on. Researchers still don’t have a good handle on which variants might cause more cases of severe disease or death, a metric called virulence. And while a virus’s potential to transmit can sometimes heighten its propensity to kill, the two are by no means inextricably linked: Future coronavirus strains could trend more lethal, or less, or neither. We keep trying to pigeonhole specific variants as “more dangerous,” “more deadly,” or “more problematic,” but viral evolution is a humbling, haphazard mess—a plot-twisting story we have to watch play out in real time. “We cannot be complacent about ‘Oh, this is the end of the mutations,’” Akiko Iwasaki, a virologist and immunologist at Yale, told me.




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