1.1.1. Susceptibility to the pandemic influenza virus will be
universal.
1.1.2. Efficient and sustained person-to-person
transmission signals an imminent pandemic.
1.1.3. The
clinical disease attack rate will likely be 30% or higher 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.
1.1.3.1. 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.
1.1.4. Of those who become ill with influenza, 50% will seek
outpatient medical care.
1.1.4.1. With the availability of effective antiviral drugs for
treatment, this proportion may be higher in the next pandemic.
1.1.5. The number of hospitalizations and deaths will depend on the
virulence of the pandemic virus. Estimates differ about 10-fold
between more and less severe scenarios. Planning should include the
more severe scenario.
1.1.5.1. 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 medical
conditions.
1.1.6. Rates of absenteeism will depend
on the severity of the pandemic.
1.1.6.1. 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.
1.1.6.2. Certain public health
measures (closing schools, quarantining household
contacts of infected individuals, “snow days”) are
likely to increase rates of absenteeism.
1.1.7. The typical incubation period (interval between
infection and onset of symptoms) for influenza is
approximately 2 days.
1.1.8. Persons who become ill
may shed virus and can transmit infection for up to one day
before the onset of illness. Viral shedding and the risk of
transmission will be greatest during the first 2 days of
illness. Children usually shed the greatest amount of virus
and therefore are likely to post the greatest risk for
transmission.
1.1.9. On average, infected persons will
transmit infection to approximately two other people.
1.1.10. In an affected community, a pandemic outbreak will last
about 6 to 8 weeks.
1.1.11. Multiple waves (periods during
which community outbreaks occur across the country) of
illness could occur with each wave lasting 2-3 months.
Historically, the largest waves have occurred in the fall
and winter, but the seasonality of a pandemic cannot be
predicted with certainty.
