The Delta variant of the coronavirus, now dominant in the U.S. and many other countries, may induce a viral load that is 1,000 times higher than the level that was typically associated with the ancestral lineage of the virus in early 2020. This higher viral load makes Delta infections more contagious, which has led to a greater number of breakthrough cases, albeit many mild or asymptomatic. But the mRNA-based vaccines still provide strong protection, with efficacy against symptomatic infection in the range of 80 to 90 percent. And the ability of these vaccines to substantially reduce viral load in breakthrough cases could be a valuable tool in containing spread of the Delta variant.