Why did COVID-19 cases rise among young adults last summer?

Last summer, as the pandemic wore on, a concerning trend emerged among young adults. A new report tracking weekly COVID-19 cases found a national increase among people aged 18 to 22 in the United States during August and early September of 2020. This kind of report, which simply observes what happened without testing an intervention, gives us a snapshot of where the virus was spreading at a specific time. It points to this age group as one where cases were climbing. However, it's important to remember this is just an observation. The report doesn't tell us why cases went up—whether it was due to social gatherings, returning to college, or other factors. It also can't establish cause and effect or predict what might happen next. It simply shows us a pattern that was happening at that moment in the pandemic.