In most cases, they found, the chain of transmission stopped with the infected child, but in 27.3 percent of households, children passed the virus along to at least one other resident.
Adolescents were most likely to bring the virus into the home: Children from 14 to 17 made up 38 percent of all the index cases. Children who were 3 or younger were the first to get sick in just 12 percent of households — but they were the most likely to spread the virus to others in their homes. The odds of household transmission were roughly 40 percent higher when the infected child was 3 or younger than when they were between 14 and 17.
The findings may be the result of behavioral differences between toddlers and teenagers, medical experts said.
“When we think about what’s teen social behavior outside of the house, they’re spending a lot of time together, they’re often in quite close quarters, they’re often touching or sharing a drink,” said Dr. Susan E. Coffin, an infectious disease specialist at Children’s Hospital of Philadelphia, who was not involved in the study.
Those behaviors could make teens more likely to contract the virus and bring it home, she said.
On the other hand, while very young children probably have less social interaction outside the home, they tend to be in close physical contact with others in their households, in addition to frequently putting their hands and other objects in their mouths, which could help spread the virus. “Once they bring it into the household, it can be spread easily,” Dr. Coffin said.
It is also possible that the youngest children may carry higher levels of virus, or have higher rates of viral shedding, than teenagers, the researchers noted. Some studies have found that even though young children rarely get seriously ill, they may carry similar, or even higher, levels of virus than adults do. Although viral load is not a perfect predictor of infectiousness, the data suggest that children could potentially be as contagious as adults.
But the dynamics of disease transmission are complicated, and the precise role that children play in spreading the virus remains uncertain.