CDC Pandemic Mitigation Guide
They split pandemics into 5 categories:
Category 1: <0.1% fatality rate, <90k deaths Category 2: 0.1%-0.5% fatality rate, 90k-450k deaths Category 3: 0.5%-1% fatality rate, 450k-900k deaths Category 4: 1-2% fatality rate, 900k-1.8M deaths Category 5: >2% fatality rate, >1.8M deaths
Lockdown policies are "Generally not recommended" for category 1. Lockdown is advised to be "Considered" for category 2-3, and "Recommended" for category 4-5.
School shutdowns are advised to "Consider <= 4 weeks" for category 2-3, and "Recommended <= 12 weeks" for category 4-5.