Influenza and the risk of active tuberculosis occurrence among individuals with latent tuberculosis infection: A national cohort study in South Korea (2015-2020)

Background Influenza's potential impact on active tuberculosis (TB) development has been debated, with limited clinical evidence. To address this, we explored the association between influenza episodes and TB incidence in a national cohort of individuals with latent TB infection (LTBI). Methods We examined adults (≥20 years) diagnosed with LTBI between 2015 and 2020, using the Health Insurance Review and Assessment Service's national database in South Korea. We collected demographic data, comorbidities, and influenza episodes within 6 months before and after the initial LTBI diagnosis (prior vs. subsequent episode). We stratified the analysis into groups with and without TB preventive therapy (TPT). Results Among 220,483 LTBI subjects, 49% received TPT, while 51% did not. The average age was 48.4 years, with 52% having comorbidities. A prior and subsequent influenza episode was identified in 3221 and 4580 individuals, respectively. Of these, 1159 (0.53%) developed incident TB over an average follow-up of 1.86 years. The incidence rates of TB were comparable between individuals with and without prior and/or subsequent influenza episodes in the TPT group, but 1.4 times higher in the non-TPT group for those with such episodes. Cox proportional-hazards regression analysis indicated that influenza was not a risk factor for incident TB in the TPT group. However, a subsequent influenza episode significantly increased TB risk in the non-TPT group (hazard ratio: 1.648 [95% CI, 1.053-2.580]). Conclusions In individuals with LTBI not receiving TPT, experiencing an influenza episode may elevate the risk of developing active TB.