The warning comes as suburban Cook County recorded a seven-day average of daily cases at 498, a spike mostly driven by the 20s, 30s, 50s and 40s age groups, in that order, according to this week’s figures. The positivity rate was 5% as of last week, creeping toward the 8% benchmark that last fall shut down indoor dining, among other things. But the higher rates reported among young adults — 6% for those under 20 — is “extremely worrisome,” Rubin said.