Teenagers are about twice as likely to become infected with the coronavirus as younger children, according to an analysis released Monday by the Centers for Disease Control and Prevention.
The report is based on a review of 277,285 cases among children aged 5 to 17 whose illness was diagnosed from March to September. The findings come as 56 million children in the country resume schooling amid contentious debates about their safety.
Scientists are scrambling to understand how often children are infected and how often they transmit the virus, but the findings have been inconsistent. Much of the national debate has centered on children in primary schools.
But the new study adds to a body of evidence suggesting that older teenagers, in high school and college, are more likely to be infected and more likely to transmit the coronavirus than are children under age 10, said Dr. Muge Cevik, an infectious disease expert at the University of St. Andrews in Scotland.
“Less emphasis had been put forward on high schools or universities, compared to younger classes, but I think that may be much more of a problem,” she said.
Children often have mild symptoms, if any, so researchers have suggested that the low reported numbers of confirmed cases in children may result from a lack of access to testing.
In support of this idea, the incidence of infections in children climbed as tests became more widely available, the C.D.C. analysis found.
The number of children tested increased to 322,227 on July 12 from 100,081 on May 31; the incidence of children found to be infected rose to 37.9 per 100,000 children from 13.8 per 100,000.
“It’s not necessarily that the incidence in children has gone up,” said Helen Jenkins, an expert in infectious diseases and statistics at Boston University. “It’s just that our testing has improved.”
Yet the dissimilar rates of infection between younger children and adolescents may partly be explained by testing. “If adolescents are more likely to have symptomatic disease, then they will be more likely to get tested,” Dr. Jenkins said. That may have led to greater numbers of confirmed cases among adolescents.
In the C.D.C. study, 58 percent of school-aged children with confirmed infections reported at least one symptom; only 5 percent reported having no symptoms. Information on symptoms was missing for 37 percent of the children.
“The chances are that this is just catching kids that are symptomatic,” said Dr. Megan Ranney, an expert in adolescent health at Brown University in Providence, R.I.
Other studies have suggested that half of children infected with the coronavirus have no symptoms. The number of cases among children could be twice as high as those reported by the C.D.C., Dr. Ranney suggested.
The incidence of confirmed coronavirus infections increased threefold among those under age 19 from May to July, and was highest among young adults 20 to 29 years old.
These data suggest that “young persons might be playing an increasingly important role in community transmission,” the C.D.C. researchers wrote.
The number of tests and the incidence of infections decreased after July, but may again be on the rise, they added.
The rate of infection during the study period varied by age. The weekly incidence among adolescents was 37.4 per 100,000, compared with 19 per 100,000 among younger children.
Children who had pre-existing medical problems were more likely to become severely ill, the analysis also found. Among school-aged children who were hospitalized, who were admitted to an intensive care unit or who died, 16 percent, 27 percent and 28 percent, respectively, had at least one underlying medical condition.
But many of the children had no such conditions. By comparison, about half of children who die after getting the flu have an underlying condition.
In the case of the coronavirus, “a kid doesn’t necessarily have to be sick already to die,” Dr. Ranney said. “They don’t have to have a pre-existing condition in order to get really sick from Covid-19,” the illness caused by the coronavirus.
Of 161,387 infected children whose race and ethnicity were known, 42 percent were Hispanic, 32 percent were white and 17 percent were Black, the C.D.C. researchers reported. Deaths among children were rare over all, but Hispanic and Black children were more likely to be hospitalized or admitted to an I.C.U.
These data are consistent with studies among adults showing that communities with a high proportion of frontline workers are hardest hit by the pandemic, Dr. Cevik said.
How often children are infected and how efficiently they transmit the virus have been among the most contentious issues of the pandemic.
Recently in JAMA Pediatrics, researchers reviewed 32 studies worldwide comprising 41,640 children and adolescents under the age of 20, as well as 268,945 adults. The analysis also included 18 studies, including three based in schools, in which scientists had traced the contacts of infected individuals.
The analysis found — like the new C.D.C. study — that younger children are roughly half as likely as adults to become infected, and that children older than 14 may be just as likely as adults to be infected. Antibody studies also suggested that adolescents seemed to be similar to adults in terms of their risk of infection.
Experts praised the scientists for trying to make sense of studies that vary widely in methods, in cultural milieu and even in how they defined children — ranging anywhere from 10 years to 20 years as the outer limit.
The evidence over all from these studies suggested that younger children are relatively protected from the virus, but older teenagers — those aged 15 and older — are roughly at the same risk as adults, said Dr. Cevik, who led a similar analysis.
”I think we need to consider the older adolescent group, over the age of 15, as an adult, because they have similar social patterns,” she said. “Also, they have potentially much larger networks, compared to adults.”
Other experts said it was clear that younger children could transmit the virus, even if less efficiently than older teenagers and adults — and thus might help perpetuate an outbreak, particularly in communities with high levels of infections.
“We know that they can get the virus,” said Jeffrey Shaman, an epidemiologist at Columbia University in New York, referring to children. “And if we know that they are able to pass it on, if we presume that they’re not complete dead ends, then they’re participating in the transmission cycle.”
Dr. Shaman pointed to a study published in May in the journal Science that suggested that children might be only one-third as efficient as adults at spreading the virus.
But when school is in session, they have three times as many contacts with others. Because of this, the risk that children will transmit the virus may resemble that of adults.
Even so, that may not be sufficient to justify keeping children at home, some scientists said.
“I don’t think that the policy conclusion is that putting kids in school is the safe endeavor,” Dr. Shaman said. “You do it under the proposition that the educational, social and emotional needs are important enough that they have to be met, in spite of the risk associated with infectious disease.”
At the least, the new studies reinforce how important it is for schools to maintain safety precautions to keep the virus from spreading, Dr. Ranney added.
“That does not necessarily mean that schools need to be closed, but rather that universal masking and then attention to all the other public health measures are just so critical, because these kids are going to be vectors,” she said.
Given that schools are open in many parts of the country, the federal government could mandate reporting of all data from schools and school districts to gather evidence on transmission from children, Dr. Jenkins said.
“We have hundreds of natural experiments going on at the moment across this country,” she said. “And it’s so sad that we are not collecting that data systematically, because we could learn so much about what works and what doesn’t.”