Federal implementation plan: Pandemic flu assumptions

The federal implementation plan, released in May, provided the following set of assumptions that underpin the planning efforts at every level:

Pandemics are unpredictable. While history offers useful benchmarks, there is no way to know the characteristics of a pandemic virus before it emerges. Nevertheless, we must make assumptions to facilitate planning efforts. Federal planning efforts assume the following:

1. Susceptibility to the pandemic influenza virus will be universal.

2. Efficient and sustained person-to-person transmission signals an imminent pandemic.

3. The clinical disease attack rate will be 30% in the overall population during the pandemic. Illness rates will be highest among school-aged children (about 40%) and decline with age. Among working adults, an average of 20% will become ill during a community outbreak.

4. Some persons will become infected but not develop clinically significant symptoms. Asymptomatic or minimally symptomatic individuals can transmit infection and develop immunity to subsequent infection.

5. While the number of patients seeking medical care cannot be predicted with certainty, in previous pandemics about half of those who became ill sought care. With the availability of effective antiviral medications for treatment, this proportion may be higher in the next pandemic.

6. Rates of serious illness, hospitalization, and deaths will depend on the virulence of the pandemic virus and differ by an order of magnitude between more and less severe scenarios. Risk groups for severe and fatal infection cannot be predicted with certainty but are likely to include infants, the elderly, pregnant women, and persons with chronic or immunosuppressive medical conditions.

7. Rates of absenteeism will depend on the severity of the pandemic. In a severe pandemic, absenteeism attributable to illness, the need to care for ill family members, and fear of infection may reach 40% during the peak weeks of a community outbreak, with lower rates of absenteeism during the weeks before and after the peak. Certain public health measures (closing schools, quarantining household contacts of infected individuals, "snow days") are likely to increase rates of absenteeism.

8. The typical incubation period (interval between infection and onset of symptoms) for influenza is approximately two days.

9. Persons who become ill may shed virus and can transmit infection for one-half to one day before the onset of illness. Viral shedding and the risk of transmission will be greatest during the first two days of illness. Children will play a major role in transmission of infection, as their illness rates are likely to be higher, they shed more virus over a longer period of time, and they don't control their secretions as well.

10. On average, infected persons will transmit infection to approximately two other people.

11. Epidemics will last six to eight weeks in affected communities.

12. Multiple waves (periods during which community outbreaks occur across the country) of illness are likely to occur with each wave lasting two to three months. Historically, the largest waves have occurred in the fall and winter, but the seasonality of a pandemic cannot be predicted with certainty.