The importance of Haemophilus influenzae in community-acquired pneumonia: an emerging pathogen in the elderly regardless of comorbidities compared to Streptococcus pneumoniae.

Background: Haemophilus influenzae community-acquired pneumonia (CAP) is common, and it is equally common to Streptococcus pneumoniae in some settings. The purpose of this study was to provide additional data on patients affected by H. influenzae CAP and their outcomes.

Methods: Streptococcus pneumoniae-caused CAP (111 cases) was compared to CAP with H. influenzae (53 cases). Patients were adults (≥ 18 years) from the prospective study "Etiology of community acquired pneumonia in Sweden" (ECAPS), which was established during the years 2016-2018.

Results: Cases with H. influenzae CAP were significantly older compared to S. pneumoniae CAP (median 77 vs 70 years, p = 0.037) albeit similar comorbidities. Haemophilus influenzae was generally absent in the bloodstream compared to S. pneumoniae (18% vs 2%, p = 0.01) but clinical presentations were comparable. Only a minority of patients, 34% with H. influenzae and 41% with S. pneumoniae CAP had underlying lung disease.

Conclusion: In the light of childhood immunization campaigns against S. pneumoniae and the increasing numbers of pneumococcal vaccinations among the elderly, coupled with an aging population, the incidence of CAP caused by H. influenzae may increase. Further research is needed to understand the impact of H. influenzae CAP and to a development of a vaccine against this emerging microbe.