Dr. Rochelle Walensky was stunned. Working from her home outside Boston on a Friday night in late July, the director of the Centers for Disease Control and Prevention had just learned from members of her staff that vaccinated Americans were spreading the coronavirus.

Vaccines had been the core of President Joe Biden’s pandemic strategy from the moment he took office. But as Walensky was briefed about a cluster of breakthrough cases in Provincetown, Massachusetts, the reality sank in. The delta variant, which had ravaged other parts of the world, was taking hold in the United States. And being vaccinated would not, it turned out, prevent people from becoming infected with the variant or transmitting it.

It was a "heart sink” moment, Walensky recalled in a recent interview. The discovery called into question the Biden administration’s almost single-minded focus on vaccination as the path out of the pandemic. And it made Biden’s July 4 message that the nation had moved "closer than ever to declaring our independence from a deadly virus” sound naive.