Chest X-ray of a man with Streptococcus pneumoniae pneumonia.

Analysis of a clinical trial just published in Nature Medicine suggests that bacterial and viral cases of pneumonia might not be as independent as previously believed.

Acute respiratory infections kill more than two million children each year and are the leading cause of death in children under five years of age. Although Streptococcus pneumoniae remains the most common bacterial cause of lower-respiratory illness in children, viral pathogens also dominate as a common cause of these infections. From previous analysis of a clinical trial to evaluate the efficacy of a 9-valent pneumococcal conjugate vaccine, it was shown that this vaccine reduced the incidence of radiologically confirmed pneumonia and also reduced the incidence of vaccine-serotype and antibiotic-resistant invasive pneumococcal disease among children. In this study, Keith Klugman and colleagues addressed the question of whether the vaccine had any effect on virus-associated pneumonia. Although earlier studies had demonstrated temporal associations between viral infections and bacterial pneumonia, the hypothesis that viruses and pathogens were co-pathogens in the disease had not been studied in the context of a randomized clinical trial.

To investigate the link between pneumonia caused by the bacterial pathogen and cases of the disease with a viral aetiology, the authors extended their analysis of the clinical trial data to show that the conjugate vaccine prevented 31% of pneumonias associated with any of seven respiratory viruses including influenza A and respiratory syncytial virus. By contrast, no differences were found in the frequency of all-cause or virus-specific bronchiolitis between children who received the vaccine and those who received the placebo. These data clearly demonstrate that the conjugate vaccine reduces pneumonia associated with respiratory viral infections, indicating that S. pneumoniae has an important role in the development of pneumonia associated with viruses and that these viruses contribute to the pathogenesis of bacterial pneumonia.

In conclusion, this study establishes that a significant proportion of viral pneumonia is attributable to bacterial co-infection and is preventable by a bacterial vaccine. Because immunization of children has been shown to reduce invasive S. pneumoniae disease, these data also raise the exciting possibility that infant immunization with pneumococcal vaccines might also reduce morbidity and mortality associated with viral pneumonias in children and adults.