Finger length ratio may help indicate vulnerability to COVID, according to a new study.
The findings suggest that low testosterone, particularly in older men, may be linked to more severe COVID-19.
In many populations, COVID appears to affect men more severely than women, explain Professor John Manning of Swansea University, UK, and colleagues.
Writing in *Scientific Reports* recently, they outline two possible reasons for this trend, either that COVID may be partly driven by androgens because viral entry to cells is androgen dependent, or that COVID severity is affected by differing exposure to testosterone and oestrogens during gestation.
The team carried out a study to see whether markers for prenatal sex hormones and postnatal stressors – evidenced by finger length ratios between the second and fourth digits – are predictive of hospitalisation for COVID.
They write: "The relative lengths of the second digit and fourth digit (or 2D:4D) is sexually dimorphic (lower in males than females)."
In tests of 54 COVID patients and 100 similar health people, they found that patients had a higher 2D:4D ratio than the control group. This group also had shorter little fingers relative to their other digits, and greater digit asymmetry between hands.
"Digit ratio patterns differ between patients and controls and this was most evident in ratios that included the fifth digit," they conclude.
"Large right left asymmetries in the patients are likely to be a marker for postnatal stressors resulting in developmental perturbations and for potential severity of COVID-19."
Professor Manning said: “Our findings suggest that COVID-19 severity is related to low testosterone and possibly high oestrogen in both men and women.
"This supports the theory that individuals who have experienced low testosterone and/or high oestrogen are prone to severe expression of COVID. This may explain why the most at-risk group is elderly males."
Kasielska-Trojan, A. et al. Digit ratios and their asymmetries as risk factors of developmental instability and hospitalization for COVID-19. *Scientific Reports* 17 March 2022; doi: 10.1038/s41598-022-08646-7
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