Background: Considering the hypothesis of cross-immunity between Mycobacterium tuberculosis and SARS-CoV-2, this study aimed to determine the association between a positive tuberculosis skin test (PPD) as a marker for latent tuberculosis infection (LTBI) and COVID-19 disease severity.
Methods: This cross-sectional analytical study was conducted on 90 patients with confirmed COVID-19 who had been referred to Al-Zahra Hospital in Isfahan. Patients were divided into three groups: mild, moderate, and severe disease (30 patients in each group) according to standard guidelines. A tuberculin skin test (PPD) was performed for all participants, and a positive result was defined as an induration size ≥10 mm. Data were analyzed using SPSS version 24 and the chi-square test.
Findings: Among the 90 patients studied, 19 (21.1%) had a positive PPD test. Data analysis showed a statistically significant association between a positive PPD test result and the severity of COVID-19 disease (p < 0.001). 94.7% (18 out of 19) of patients with positive PPD were in the mild disease group. In contrast, none of the patients with severe COVID-19 (out of 30) had a positive PPD test. Furthermore, the presence of a BCG vaccine scar was not statistically significantly associated with the severity of COVID-19 disease (P = 0.9).
Conclusion: The results of the study suggest that latent tuberculosis infection (based on a positive PPD skin test) may be associated with the occurrence of milder forms of COVID-19. This may indicate a potential protective role of cellular immunity induced by Mycobacterium tuberculosis against the severity of SARS-CoV-2 infection.