As seen at WSJ.com (subscriber link):
Taking a cue from hurricane forecasters, federal officials unveiled a system to predict the severity of the next world-wide outbreak of a super flu — a guide to help states determine when they should think about taking increasingly strong steps to combat its spread.[...]
If the CDC decides a new flu strain will be the least severe, with the potential of killing no more than 90,000 people, states would be urged to just isolate the sick, mostly at home, minimizing visitors who might catch the virus.
With a more moderate 1957- or 1968-like strain — 90,000 to 450,000 potential deaths — states might consider closing schools, but CDC doesn’t recommend it and says any closures should be less than a month. If more than 900,000 deaths are predicted, a Category 4 or 1918-like Category 5 pandemic, then CDC does recommend closing schools, from one month to three months, depending on each community’s level of illness. Other steps are recommended too, including avoiding crowded gatherings and encouraging people to work from home.
Information on the severity index can be found at pandemicflu.gov.