Kids spend the majority of their time around adults, and existing contact-tracing data suggest that adults are the ones getting kids sick. “There is with Delta, we think, a reasonably high household attack rate, meaning that one person in the household gets sick and other people are at risk of getting sick,” says Ashish Jha, the dean of the Brown University School of Public Health.

COVID-19 outbreaks are larger in under-vaccinated areas, so it stands to reason that kids in those areas would come into contact with more COVID-infected adults. That’s exactly what the numbers show: COVID-19 rates among kids appear to be rising in states where fewer adults are vaccinated. Among the states with the largest recent increases in child COVID-19 cases, according to the most recent report from the American Academy of Pediatrics, are Louisiana, Missouri, Arkansas, and Florida—states where relatively few adults are vaccinated.

Conversely, vaccinating more adults and older children seems to decrease the number of COVID-19 cases among younger kids. In Israel, COVID-19 cases in unvaccinated kids plummeted after adults got vaccinated in large numbers earlier this year—even though schools reopened in March. One study, also out of Israel, found that every 20-point increase in adult vaccination rates in a community halved the number of kids testing positive for COVID-19. “Every time somebody gets vaccinated, everybody around them becomes a little more protected,” Jha says.