While there are similarities between all human influenza (e.g. modes of transmission and how transmission risk can be reduced) here ECDC lists ten differences between the old (2000/1 to 2008/9) seasonal influenza and the 2009 pandemic.

	Seasonal Influenza 2000/1 to 2008/9	2009 Pandemic Influenza
Circulating Influenza Viruses	Two A viruses (H1N1), (H3N2) & some B viruses - blend varied with season	Almost exclusively the pandemic (H1N1), a few (H3N2) & increasing numbers of B viruses latterly
When waves occurred	In season - mostly starting after Christmas in recent years	Started out of season with Spring/Summer wave then an early Autumn/Winter wave in Europe
Intensity of transmission	Variable year on year, with local heterogeneity but estimated to be 5 to 15% per annum	Hard to estimate, local heterogeneity, estimated to be over 15% through serological studies reviewed by WHO and in New Zealand
Setting for transmission	Probably any setting where people come together	Schools are considered especially important, along with household transmission
Experiencing severe disease	Those in clinical risk groups and older people	Young children, pregnant women and those in clinical risk groups.  About 30% with severe disease were outside risk groups.  Many born before mid-1950s were immune, but those not experienced severe disease. Donaldson et al
Premature Deaths	Considered that around 90% were in those aged 65 years or older Thompson et al	In confirmed reported deaths around 80% were under age 65 years (Reports to ECDC)    Increase in all-cause deaths in children detected across eight EU countries by Euromomo system
Mortality and Years of Potential Life Lost (YPLL)	Few confirmed deaths reported each year in official statistics. Have been estimates of up to 40,000 in a bad year using  statistical methods	Substantial numbers of confirmed deaths announced by EU/EFTA Member States (n = 2900) but recognised to be an underestimate. Not estimated in any EU member states but estimated in the US
Acute respiratory distress syndrome	Extremely rare	Uncommon but has been recorded in many countries, even in young fit adults. WHO Clinical summary
Pathological Findings	Viral pneumonia rare but secondary bacterial infections more common in fatal cases	Fatal viral pneumonias relatively common with alveolar lining cells, including type I and type II pneumocytes the primary infected cells. In fatalities more than 25% also had bacterial infections. WHO Clinical summary  Shieh et al
Antiviral resistance	Common and transmissible oseltamivir resistance in A(H1N1) emerged in season 2007/8 Meijer et al	Observed most often following antiviral treatment of susceptible individuals. However to date (July 2010) only transmitting very rarely in certain circumstances see WHO. Resistant seasonal A(H1N1) seemingly displaced by the new influenza, at least for now.