Invasive bacterial infections in relation to influenza outbreaks, 2006-2010

We aimed to define the excess morbidity associated with bloodstream infections (BSIs), imposed by pandemic H1N1 influenza during 2009-2010 (pH1N1/2009-2010) and seasonal influenza.

Eight hospitals, accounting for 33% of hospitalizations in Israel, provided data on BSI during 2006-2010. The age-specific incidence of BSI due to Streptococcus pneumoniae, Staphylococcus aureus, and Streptococcus pyogenes was determined. BSI incidence rate ratios (IRRs) during seasonal and pH1N1 influenza seasons were assessed.

Regular influenza seasons were characterized by increased rates of S. pneumoniae BSI but with no increase in S. aureus and S. pyogenes BSI rates. The pH1N1/2009-2010 influenza outbreak was characterized by (1) higher rates of S. pneumoniae bacteremia among children but not among adults (IRRs for S. pneumoniae BSI among children aged 0-4 years during the summer and winter of 2009-2010 were 14.8 [95% confidence interval {CI}, 5-43.7] and 6.5 [95% CI, 3.6-11.8], compared with 2006-2009 summers and influenza-active winter weeks, respectively [P < .0001]), higher rates of S. aureus BSI in all age groups (IRRs during the summer and winter of 2009-2010 were 1.6 [95% CI, 1.4-1.9] and 1.5 [95% CI, 1.2-1.7], compared with 2006-2009 summers and influenza-active weeks, respectively [P < .0001]), higher rates of S. pyogenes BSI during 2009-2010 influenza season (IRR 2.7 [95% CI, 1.6-4.6] and 3.3 [95% CI, 1.9-5.8] during the summer and winter of 2009-2010, compared with 2006-2009 summers and influenza-active weeks, respectively [P < .0001]).

pH1N1 influenza seasons were characterized by marked increases in invasive S. aureus and S. pyogenes infections among children and adults, with the highest increase in S. pneumoniae BSI among children.