General Assumptions

Outbreak Trends

The first outbreaks in the U.S. are likely to occur in major metropolitan areas where the rates of international and business travel are high.
Susceptibility to the virus will be universal.
The disease will likely occur in multiple waves and each wave will last between 6 to 12 weeks.
Absenteeism will be in the range of 40%.
Each wave of disease will be separated by several months
The disease will occur simultaneously throughout the U.S. (and world) and therefore outside assistance will be unlikely or limited at best.

Some persons will become infected but not develop clinical illness and will thus unknowingly transmit infection.
Each infected individual will transmit the infection to two other individuals.
Persons who become ill may shed virus and can transmit infection for several days before the onset of illness, with the highest contagion during the first two days of illness, and several days following lack of symptoms.
The attack rate will be 30% or higher in the overall population, highest in children in grades K-7 and the elderly.
The number of hospitalizations and deaths will depend on the virulence of the virus. Estimates differ tenfold between more and less severe scenarios. Planning should include the more severe scenario.
Of those infected, about 50% will seek medical care (this could be higher if antiviral medications and vaccines are available).

Because a vaccine needs to closely match an influenza virus, it is unlikely that an effective vaccine would be available early in a pandemic and quantities will be limited even when available. A vaccine will probably be available later in the pandemic (second and subsequent waves) but quantities may not meet demand and there will need to be priority lists for distribution.
Isolation, quarantine, social distancing and personal hygiene practices are likely to be at least somewhat effective in slowing transmission and thus reducing the overall impact of the disease, especially very early in the outbreak.