The coronavirus can still potentially find new ways to enhance its transmissibility

Some scientists had wondered whether Delta was so transmissible that it had reached some peak spreading capacity. Or they speculated that if Delta were to gain any new spreading prowess, it might have to cede some other feature, such as how often it causes severe disease. After all, mutation combinations that confer certain traits sometimes come at the expense of others.

But if AY.4.2 does in fact have an edge over Delta, it disproves that theory. On top of potentially being more transmissible than the original Delta strain, the preliminary evidence out of the U.K. indicates Delta Plus carries the same risk of hospitalization and death as its parent virus (though that is based on limited data).

Whether Delta itself causes more severe disease on average than earlier forms of the virus remains an open question; in some countries, researchers have reported that Delta was causing higher rates of hospitalizations among people with Covid-19, though other research — including a report last week from the Centers for Disease Control and Prevention — did not find a difference in disease severity.