Among the many mysteries about long COVID, one of the most vexing has been why women seem to experience the condition more often and more severely than men.

Now, scientists are starting to think hormones — and the different ways they affect women and men — could be part of the puzzle.

A new study by a prominent team of researchers from the Yale School of Medicine and the Icahn School of Medicine at Mount Sinai has found that women with long COVID had significantly lower levels of testosterone compared to those who had recovered from their infection. That difference seems to driving certain symptoms female patients experience more often and more severely than male patients, such as headaches, hair loss, muscle pain and memory issues. Low testosterone in women was also associated with elevated levels of distinct immune cells, as well as signs that dormant viruses had been reactivated. While the researchers found that men with long COVID had lower levels of estradiol (indicative of low testosterone), their symptoms were less burdensome and different immune cells were activated.