The following table provided by the Indiana State Department of Health illustrates the differences between pandemic influenza and typical seasonal influenza.


Pandemic Influenza  Typical Seasonal Flu 
Comes from a novel virus to which no one, or few people, have any immunity Circulating viruses slightly mutate year to year
Attack rate may be as high as 30% of population Attack rate 5-20% of population
Patients are sicker for a longer period time Illness usually lasts 1 to 2 weeks
Limited vaccine availability Vaccine available prior to illness in population
May be resistant to some antivirals Several antiviral medications are available
Excess mortality (3 to 7 times normal rate) Normal flu mortality—36,000/year in U.S.
Patients may present with primary viral pneumonia Pneumonia is usually a later complication
May be severe in all ages Severe in the very old and the very young
Can occur at any time of the year Seasonal—occurs in the winter
May have more than one wave of illness Usually just one wave of illness
Spreads rapidly throughout the world