Association of Lung cancer with Pneumonia and Chlamydia pneumoniae infection

Introduction: The degree of association and type of causal versus non-causal relationship between pneumonia and lung cancer (LC) are evolving discussions. We reviewed English- language publications on the degree of association between pneumonia and subsequent LC. Methods: We searched the PubMed database using key- words for pneumonia, LC, and Chlamydia infection. We selected peer-reviewed studies of patients with pneumonia and LC. Case reports and other literature reviews were excluded from this review. Results: Five studies examined the incidence and/or risk of LC for a total of 415,750 patients, and four studies examined cases with Chlamydia pneumoniae chronic infection at the time of diagnosis of LC for a total of 1,467 patients. The overall risk and/or incidence of LC after pneumonia was from 2.3% to 10% for a median follow-up ranging from 109 days to 4.2 years. Three studies reported current tobacco smok- ing status, which ranged from 27.7% to 45% among those with LC. A history of prior malignancy was reported in 22.5% of patients with LC. Chlamydia immunoglobulin (Ig) A and LC were statistically non-significantly associated regardless of the age of the patients. In one study, Chlamydia heat shock protein (HSP)-60 IgG ≥ 1:50 was associated with significantly increased odds of LC in two respective models (odds ratios of [95% confidence interval (CI) 1.06–1.69] and 1.30 [95% CI 1.02–1.67]). A fourth study reported C. pneumoniae IgA ≥ 64 titers to be 58%, 29%, and 5.5% among patients with LC, those without LC, and healthy blood donors, respectively. Conclusions: The incidence of LC was reported to range from 2.3% to 10.3% following an episode of pneumonia. There is limited evidence of the association of chronic Chlamydia infection with LC, and Chlamydia could be a causal cofactor of LC. causal cofactor of lung cancer. Metagenomic studies are needed to examine the changes in the lung microbiome triggering upregulation of the P13K signaling pathway.