When you look at your fingers, you may notice that your ring finger is actually longer than your index finger, which looks wildly counterintuitive. Past research has demonstrated this likely means you were exposed to more testosterone before birth, with men often having longer ring fingers than index fingers and women often having longer index fingers, and this is a relatively reliable marker of prenatal testosterone. 

However, testosterone also has some interesting implications in COVID-19 severity, as men are more likely to get severe disease when infected and some scientists believe testosterone could be the link. Is it therefore possible that the length of a person’s fingers could be a marker for COVID-19 severity. 

According to a new preliminary study it is, after data from hospitalized patients showed a significant increase in disease severity in people with what the study calls “feminized” digits compared to controls. The findings are published in the journal Scientific Reports. 

The researchers believe this correlation may add more evidence to the hypothesis that low testosterone and high estrogen, in both men and women, could be a predictor of disease severity, and could even be used as a way to identify the most at-risk groups.  

"'Feminized' differences in digit ratios in hospitalized patients supports the view that individuals who have experienced low testosterone and/or high estrogen are prone to severe expression of COVID-19. This may explain why the most at-risk group is elderly males,” said Professor John Manning of Swansea University's Applied Sports Technology, Exercise, and Medicine team, who worked in collaboration with the Medical University of Lodz in Poland and Sweden's Karolinska University Hospital, in a statement. 

"This is significant because if it is possible to identify more precisely who is likely to be prone severe COVID-19, this would help in targeting vaccination," he said. "Right-Left differences in digit ratios (particularly 2D:4D and 3D:5D) may help in this regard." 

To identify any link, the team of scientists took 54 patients hospitalized with COVID-19 and 100 controls, comparing different ratios of finger length alongside how severe their disease was. They discovered that people with shorter little fingers compared to their other fingers on the same hand experienced more severe COVID-19 than those with more typical digit ratios, and those with greater differences in their 2nd and 4th digits on either hand had significantly higher chance of hospitalization. 

The work adds to a growing body of evidence that links testosterone with COVID-19, using testosterone as both an anti-viral against COVID-19, but also inhibiting testosterone as a separate treatment to lower severity. Professor Manning and the team now hope to improve the sample size in their current work, which they will announce shortly.