Young people who contract COVID-19, but who are otherwise healthy, show a disrupted fight-or-flight response that can last for months, according to new research.

In a six-month study monitoring 30 participants (16 of whom had contracted COVID-19), what researchers ultimately found falls largely in line with what many COVID-19 'long haulers' have been reporting to date.

Even while at rest, the nerves of young, COVID-19 survivors seem to work much harder than those who have not contracted the virus.

In the study, a tiny electrode, inserted into the back of the knee, found reduced electrical activity in the muscles of those recovering from COVID-19.

These nerves receive messages from the sympathetic nervous system, which controls our fight-or-flight response, and for those who are recovering from the virus, this system appears abnormally active.

When going from lying down to tilted upright, for instance, the 16 COVID survivors showed elevated sympathetic activity, followed by an exaggerated heart rate response to the change in blood pressure.

It's worth noting that this is only a small study, but even though the cohort was tiny, the findings do match up with recent reports suggesting many long haulers – people experiencing COVID-19 symptoms for months on end – are unable to keep their standing heart rate from beating out of control.

In other cases, however, the sympathetic nervous system of COVID-19 patients appears to be seriously under-active.

When the young participants stuck their hands in ice cold water, for example, their muscle nerve activity was lower than normal and the participants reported feeling substantially less pain than 14 healthy control subjects.

This isn't a perfect comparison; it would be better to have tested survivors before and after contracting COVID-19, to see how their personal physiological responses might have changed over time. But given the surprising nature of this disease, using healthy participants who have not been exposed to the virus as a control is our next best option.

The results suggest SARS-CoV-2 can somehow throw off the sympathetic nervous system, making it work too hard at rest and not hard enough during stressful situations, even among young people who are not as vulnerable to severe illness.

"If similar autonomic dysregulation, like that found here in young individuals, is present in older adults following SARS-CoV-2 infection," the authors write, "there may be substantial adverse implications for cardiovascular health."

But for now, there's just a whole lot of 'ifs', with not a whole lot of answers.

Higher resting activity in the sympathetic nervous system is known to increase a person's heart rate and cardiac output, possibly putting stress on the cardiovascular system over time. But this is the first study to examine nervous system activity after SARS-CoV-2 infects the body.

Today, some 200 million people around the world have survived COVID-19, and if the new findings can be verified among much larger cohorts, then it means millions of people might have suffered from an abnormal sympathetic nervous system for months during their recovery.

How long those symptoms lasted before they cleared up remains unknown. The current study tracked participants for six months, but some long-haulers are still showing high standing heart rates and other cardiovascular issues long after that.

The authors are not sure what it is about SARS-CoV-2 that is disrupting the sympathetic nervous system of patients to such a degree, but they suspect oxidative stress and inflammation might be to blame.

High levels of sympathetic nervous activity in the muscles have been linked to increased arterial stiffness in the past, and interestingly enough, young adults with COVID-19 have shown greater stiffness in their arteries up to three months after testing positive for the virus.

This reduced elasticity can change how much oxygen is being carried to a person's head or heart, and this, in turn, could be kickstarting the body's fight-or-flight response. On the other hand, inflammation, brought on by the immune system, could also be triggering the sympathetic nervous system.

"If, in fact, our [COVID positive] participants also have increased resting vessel diameters, the higher resting [muscle sympathetic nervous system activity] could serve as an acute adaptation to systemic vasodilation," the authors suggest.

"Certainly, we are limited in these interpretations given the cross-sectional nature and short time-frame of this study."

It's hard to conclude much from the results, but the implications are worrisome. Given the possible health issues that can arise from an overactive sympathetic nervous system, it's important we find out more.

The study was published in the Journal of Physiology.