Influenza virusInfluenza infection increases susceptibility to pneumococcus, the most common bacterial cause of pneumonia, by about 100-fold, according to research in Science Translational Medicine.

Researchers created a computer model of pneumococcal pneumonia transmission capable of analyzing various hypotheses about the potential effects of a prior influenza infection. By challenging the model with hard data from epidemiological reports — weekly records of influenza and pneumococcal pneumonia hospitalizations in Illinois between 1989 and 2009 — they were able to rank the likelihood of each hypothesis.

The most likely explanation was explained in the susceptibility impact hypothesis, which proposed that individuals infected with influenza are more susceptible to pneumococcal pneumonia. The increased susceptibility to pneumonia lasts for up to a week after infection by influenza.

The researchers also looked at the fraction of pneumonia cases that could be attributed to interaction with influenza. They found that during the peak of flu season, interaction with the influenza virus accounted for up to 40% of pneumococcal cases. But on an annualized basis, the fraction was between 2% and 10% of cases, a relatively subtle signature that could help explain why previous epidemiological analyses failed to detect the connection, the authors concluded.

Authors noted that the results suggest the best way to reduce the incidence of bacterial pneumonia is to encourage the public to receive both pneumococcal and influenza vaccinations.