Even as many Americans celebrate the apparent waning of the pandemic, the thrum of concern over the so-called Delta variant grows steadily louder.

The variant, the most contagious version yet of the coronavirus, accounts for more than half of new infections in the United States, federal health officials reported this month. The spread of the variant has prompted a vigorous new vaccination push from the Biden administration, and federal officials are planning to send medical teams to communities facing outbreaks that now seem inevitable.