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It's still mild out, yet by the first day back, noses are running

The number of encounters takes off

On the day term starts, millions of pupils rebuild a contact network all at once

In a single day, teenagers encounter more different faces than any other group

During the holidays, even in winter, transmission to children slows sharply

School speeds up the spread

It takes several jumps before a peak becomes visible

Young children carry detectable viruses more often, and for longer

18 days: the gap measured among schoolchildren between the start of term and the peak

The school calendar calls the shots, not the weather