What is clear is that a confluence of factors – including Delta’s contagiousness and the fact that people under the age of 12 are not yet eligible for vaccination – will result in more children being hospitalized, especially in areas of the country in where the virus is increasing. “If you have more cases then of course it eventually comes down to the kids,” said Dr. Malley.

Many children’s hospitals had hoped for a quiet summer. Several common childhood viruses are less common in the warmer months, and national Covid rates declined in the spring.

But that started to change last month as Delta spread. “The number of positive Covid tests began to rise in early July,” said Marcy Doderer, president and chief executive officer of Arkansas Children’s Hospital. “And then we really started to see the kids get sick.”

The vaccines are effective against Delta – and offer strong protection from serious illness and death – but children under 12 are not yet eligible for them. As more adults are vaccinated, children make up an increasing proportion of Covid cases; between July 22 and July 29, they accounted for 19 percent of reported new cases, according to the American Academy of Pediatrics.

“They’re the unvaccinated,” said Dr. Yvonne Maldonado, a pediatric infectious disease specialist at Stanford Medicine and chair of the AAP Infectious Diseases Committee. “We see all new infections there.”

According to the association, almost 72,000 new pediatric Covid cases were reported from July 22 to July 29, almost twice as many as in the previous week. At Johns Hopkins All Children’s Hospital, 181 children tested positive for the virus in July, up from just 12 in June.

Most of these children have relatively mild symptoms such as runny nose, constipation, cough, or fever, said Dr. Wassam Rahman, the medical director of the Pediatric Emergency Center at All Children’s. “Most children are not very sick,” he said. “Most of them will go home and receive preventive treatment at home. But as you can imagine, families are scared. “