Influenza makes people vulnerable to bacterial infection


Researchers at Sweden’s Karolinska Institutet have described findings leading to so-called “superinfections”, it can also contribute to research onCovid-19.


  • Influenza is caused by a virus, but the most common cause of death in influenza patients is secondary pneumonia caused by bacteria, rather than the influenza virus itself.
  • While this is well known, what is largely unknown is why influenza infections lead to an increased risk of bacterial pneumonia.
  • The Spanish flu, which was an influenza pandemic that swept across the world in 1918–20.
  • Unlike many other pandemics, the Spanish flu disproportionately hit young healthy adults.
  • And one important reason for this was “superinfections” caused by bacteria, in particular pneumococci.
  • Pneumococcal infections are the most common cause of community-acquired pneumonia and a leading global cause of death.
  • A prior influenza virus infection is often followed by a pneumococcal infection. In the new study, the mechanisms behind this increased susceptibility: influenza induces changes in the lower airways that affect the growth of pneumococci in the lungs.
  • Research found that different nutrients and antioxidants, such as vitamin C, leak from the blood.
  • This creates an environment in the lungs that favours growth of the bacteria.
  • The bacteria adapt to the inflammatory environment by increasing the production of an enzyme called HtrA.
  • The presence of HtrA weakens the immune system and promotes bacterial growth in the influenza-infected airways.
  • The researchers suggest the results could be used to find new therapies for double infections between the influenza virus and pneumococcal bacteria.
  • A possible strategy can therefore be use of protease inhibitors to prevent pneumococcal growth in the lungs.
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