As school districts across the United States consider whether and how to restart in-person classes, they face two fundamental uncertainties: No nation has tried to send children back to school with the virus raging at levels like Americas, and the scientific research about transmission in classrooms is limited.

The World Health Organization has concluded that the virus is airborne in crowded, indoor spaces with poor ventilation, a description that fits many American schools, and many of the countrys 3.5 million teachers are feeling under siege, under pressure from the White House, pediatricians and some parents to resume normal teaching.

Im just going to say it: It feels like were playing Russian roulette with our kids and our staff, said Robin Cogan, a nurse at the Yorkship School in Camden, N.J., who serves on the states committee on reopening schools.

Three science reporters for The Times, Pam Belluck, Apoorva Mandavilli and Benedict Carey, reviewed relevant studies from around the world. The data, they write, clearly shows that children are far less likely than adults to become seriously ill from the virus. And some research suggests younger children are less likely than teenagers to infect others, though the evidence is not conclusive.

Countries like Norway and Denmark reopened schools after reducing infection levels, and have not seen a surge in cases. They initially opened only for younger children, strengthened sanitizing procedures, and have kept class size limited, children in small groups at recess and space between desks.

The larger concern is that children could become infected, many with no symptoms, and then spread the virus to others, including family members, teachers and other school employees. In Israel, the virus infected more than 200 students and staff after schools reopened in early May and lifted limits on class size a few weeks later, according to a report by University of Washington researchers.

On the other hand, a study in Ireland of six infected people two high school students, an elementary student and three adults who spent in schools before they closed in March found that the only documented transmission involved one of the adults, and outside of school.

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