Exposure to Mycobacteria influences disease progression in COVID-19 patients 

Abstract Background : COVID-19−related deaths are significantly higher in countries with higher quality of life. A strong negative correlation is reported between the BCG index and COVID- 19 mortality. The present study explored if a high Th1immunity due to frequent exposure to strong Th1 antigens like Mycobacteria or Salmonella could be the cause for lesser COVID-19−related deaths in Indian population. Methods : This prospective comparative study was conducted with 3 groups of twenty patients each of mildly symptomatic (A), severely ill (S) Covid patients and healthy volunteers with a Covid Negative report (H). Results : All severely ill patients showed increased leucocyte counts, lymphopenia and raised D-dimer. A gross reversible unresponsiveness of T cells was seen among all patients in S group with absolutely no response even to the mitogen stimulus. Quantiferon TB test value and distribution of test positivity was significantly lower in group S. Three out of 6 survived patients in S group had positive Quantiferon TB test while 2 patients turned positive on repeat test and the sixth patient showed high TH titre on widal test. Conclusion: Altered Th1 immunity associated with frequent community exposure of tuberculosis and typhoid antigen in Indian population might be responsible for its relatively lesser prevalence and mortality following Covid-19.