Use of the diabetes drug metformin -- before a diagnosis of COVID-19 -- is associated with a threefold decrease in mortality in Covid-19 patients with Type-2 diabetes, a new study suggests.

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"Since similar results have now been obtained in different populations from around the world -- including China, France and a UnitedHealthcare analysis -- this suggests that the observed reduction in mortality risk associated with metformin use in subjects with Type-2 diabetes and COVID-19 might be generalizable," said researcher Anath Shalev from the University of Alabama at Birmingham.

According to the researchers, how metformin improves prognosis in the context of COVID-19 is not known. The findings suggest that the mechanisms may go beyond any expected improvement in glycemic control or obesity, since neither body mass index, blood glucose nor hemoglobin A1C were lower in the metformin users who survived as compared to those who died.

For the study, published in the journal Frontiers in Endocrinology, the team included 25,326 patients tested for COVID-19. The primary outcome in the study was mortality in COVID-19-positive subjects, and the potential association with subject characteristics or comorbidities was analyzed.

Overall, mortality for COVID-19-positive patients was 11 per cent. The study found that 93 per cent of deaths occurred in subjects over the age of 50, and being male or having high blood pressure was associated with a significantly elevated risk of death. Diabetes was associated with a dramatic increase in mortality, with an odds ratio of 3.62. Overall, 67 per cent of deaths in the study occurred in subjects with diabetes.

The researchers looked at the effects of diabetes treatment on adverse COVID-19 outcomes, focusing on insulin and metformin as the two most common medications for Type-2 diabetes. They found that prior insulin use did not affect mortality risk. However, prior metformin use was a different matter. Metformin use significantly reduced the odds of dying, and the 11 per cent mortality for metformin users was not only comparable to that of the general COVID-19-positive population, it was dramatically lower than the 23 per cent mortality for diabetes patients not on metformin.