During the COVID-19 pandemic, some studies attempted to establish a correlation between selenium plasma level and severity of COVID-19 cases. One study done on 33 patients concluded that low plasma selenium levels were correlated with a high mortality rate among COVID-19 patients. However, the median age of deaths in this study was 89 years old; in contrast, survivors' median age was 69 years old, and the study stated that the causality remains unknown.  
On the other hand, another study revealed that the mean selenium plasma level was within the normal range among all included COVID-19 patients; however, the mean selenium plasma level was elevated among severe cases of COVID-19. This study concluded that there was a significant elevation of selenium serum level among severe cases compared to non-severe cases of COVID-19, and could be correlated with the disease severity.