Letícia Soares was infected with COVID-19 in April 2020, in the final year of postdoctoral studies in disease ecology at a Canadian University. What started with piercing migraines and severe fatigue in 2020 soon spiraled into a myriad of long COVID symptoms: Gastrointestinal issues, sleep problems, joint and muscle pain, along with unexpected menstrual changes.
After an absence of menstrual bleeding and its usual signs, she later suffered from severe periods and symptoms that worsened her long COVID condition. “It just baffled me,” said Soares, now 39. “It was debilitating.”
Cases like Soares’s are leading scientists to spend more time trying to understand the biological sex disparity in chronic illnesses such as long COVID that until recently have all but been ignored. According to the Centers for Disease Control and Prevention, long COVID affects nearly twice as many women as men.
What’s more, up to two thirds of female patients with long COVID report an increase in symptoms related to menstruation, which suggests a possible link between sex hormone fluctuations and immune dysfunction in the illness.
“These illnesses are underfunded and understudied relative to their disease burdens,” said Beth Pollack, a research scientist at the Massachusetts Institute of Technology, Cambridge, Massachusetts, who studies complex chronic illnesses.
Addressing knowledge gaps, especially around sex differences, could significantly improve our understanding of complex chronic illnesses, said Pollack, who coauthored a 2023 literature review of female reproductive health impacts of long COVID.
Emerging ‘Menstrual Science’ Could Be Key
There is a critical need, she said, for studies on these illnesses to include considerations of sex differences, hormones, reproductive phases, and reproductive conditions. This research could potentially inform doctors and other clinicians or lead to treatments, both for reproductive symptoms and for the illnesses themselves.
Pollack noted that reproductive symptoms are prevalent across a group of infection-associated chronic illnesses she studies, all of which disproportionately affect women. These associated conditions, traditionally studied in isolation, share pathologies like reproductive health concerns, signaling a need for focused research on their shared mechanisms.
Recognizing this critical gap, “menstrual science” is emerging as a pivotal area of study, aiming to connect these dots through focused research on hormonal influences.
Researchers at the University of Melbourne, Melbourne, Australia, for example, are studying whether hormones play a role in causing or worsening the symptoms of long COVID. By comparing hormone levels in people with these conditions with those in healthy people and by tracking how symptoms change with hormone levels over time and across menstrual cycles, scientists hope to find patterns that could help diagnose these conditions more easily and lead to new treatments. They’re also examining how hormonal life phases such as puberty, pregnancy, or perimenopause and hormone treatments like birth control might affect these illnesses.
How Gender and Long COVID Intertwine
The pathologies of long COVID, affecting at least 65 million people worldwide, currently focus on four hypotheses: Persistent viral infection, reactivation of dormant viruses (such as common herpes viruses), inflammation-related damage to tissues and organs, and autoimmunity (the body attacking itself).
It’s this last reason that holds some of the most interesting clues on biological sex differences, said Akiko Iwasaki, PhD, a Yale University, New Haven, Connecticut, immunologist who has led numerous research breakthroughs on long COVID since the start of the pandemic. Women have two X chromosomes, for example, and although one is inactivated, the inactivation is incomplete.
Some cells still express genes from the “inactivated genes” on the X chromosome, Iwasaki said. Those include key immune genes, which trigger a more robust response to infections and vaccinations but also predispose them to autoimmune reactions. “It comes at the cost of triggering too much immune response,” Iwasaki said.
Sex hormones also factor in. Testosterone, which is higher in males, is immunosuppressive, so it can dampen immune responses, Iwasaki said. That may contribute to making males more likely to get severe acute infections of COVID-19 but have fewer long-term effects.
Estrogen, on the other hand, is known to enhance the immune response. It can increase the production of antibodies and the activation of T cells, which are critical for fighting off infections. This heightened immune response, however, might also contribute to the persistent inflammation observed in long COVID, where the immune system continues to react even after the acute infection has resolved.