Invasive bacterial infections following influenza: a time‐series analysis in Montréal, Canada, 1996–2008

BACKGROUND Shared seasonal patterns, such as between influenza and some respiratory bacterial infections, can create associations between phenomena not causally related. OBJECTIVES To estimate the association of influenza with subsequent bacterial infections after full adjustment for confounding by seasonal and long-term trends. METHODS Time series of weekly counts of notified cases of invasive infections with Haemophilus influenzae, Neisseria meningitidis, Streptococcus pneumoniae and Streptococcus pyogenes, in Montréal, Canada, 1996-2008, were modelled by negative binomial regression, with terms representing seasonal and long-term trends and terms for numbers of positive laboratory tests for influenza A and B. RESULTS The associations of S. pneumoniae, H. influenzae and N. meningitidis with influenza disappeared after seasonal terms were added to the model. However, the influenza B count remained associated with the S. pyogenes counts for the same week and the following week: S. pyogenes incidence rate ratios were 1.0376 (95% CI: 1.0009-1.0757) and 1.0354 (0.9958-1.0766), respectively, for each increase of 1 in the influenza count. CONCLUSIONS Influenza B accounts for about 8 percnt; of the incidence of invasive S. pyogenes infections, over and above any effect associated with modellable seasonal and long-term trends. This association of influenza B with S. pyogenes infections can be attributed largely to the years 1997, 2001, 2007 and 2008, when late peaks in influenza B counts were followed by peaks in S. pyogenes notifications. This finding reinforces the case for universal immunization against influenza, as partial protection against the 'flesh eating disease'.