Seeing infection levels dropping in the spring, and determined to reboot the economy, Israel retired its electronic pass system, eased travel bans and lifted all other restrictions. The last to go was the indoor mask mandate on June 15.

Days earlier, however, a family had returned from a Greek vacation to the central city of Modiin, a middle class commuter hub between Tel Aviv and Jerusalem. More than 90 percent of its residents 12 and over are vaccinated, according to its mayor, Haim Bibas, making it one of Israel’s most vaccinated cities.

But the family included a child too young to be vaccinated, and who should have spent at least 10 days in home quarantine pending a negative PCR test, according to the regulations at the time.

Instead, the parents sent the child to school. Ultimately, about 80 students were infected with the Delta variant.

“The child wasn’t to blame,” Mr. Bibas said, indirectly pointing a finger at the parents.