H1N1 infections in 2009 were more likely to be in younger people than seasonal flu strains, researchers found.
But hospitalisation of children with 2009 H1N1 was no more common than among those with seasonal flu.
Scientists believe the findings reinforce the need for active influenza surveillance in a defined group to assess the ongoing severity of a pandemic.
The team from Marshfield Clinic Research Foundation, Wisconsin, assessed 545 patients with 2009 H1N1, 221 with seasonal H1N1 and 632 with H3N2.
Researchers followed up patients for 30 days to assess complications. Hospital admission in children was similar among those infected with 2009 H1N1 as with seasonal strains of H1N1 and H3N2.
However, the risk of pneumonia in adults with 2009 H1N1 was greater than for those with seasonal flu strains.
The research is published in JAMA.