Why Covid-19 Vaccines Work Well Against Delta Variant

Covid-19 vaccines, designed to target the earliest version of the coronavirus, are holding up well against the Delta variant because it isn’t so different from the original strain that it can evade the shots, researchers said.

Yet the Delta variant is different enough, the researchers said, that studies indicate people will need both doses of the messenger RNA shots or a booster for the one-dose vaccines to get the kind of immune response necessary to provide full protection.

The Delta variant is the most contagious version of the virus to be identified. In a...